connecting to a service provider using external bgp · bgp (ebgp) peering sessions are configured...

Connecting to a Service Provider UsingExternal BGP

Last Updated: September 8, 2011

This module describes configuration tasks that will enable your Border Gateway Protocol (BGP) networkto access peer devices in external networks such as those from Internet service providers (ISPs). BGP is aninterdomain routing protocol that is designed to provide loop-free routing between organizations. ExternalBGP (eBGP) peering sessions are configured to allow peers from different autonomous systems toexchange routing updates. Tasks to help manage the traffic that is flowing inbound and outbound aredescribed, as are tasks to configure BGP policies to filter the traffic. Multihoming techniques that provideredundancy for connections to a service provider are also described.

• Finding Feature Information, page 1

• Prerequisites for Connecting to a Service Provider Using External BGP, page 2

• Restrictions for Connecting to a Service Provider Using External BGP, page 2

• Information About Connecting to a Service Provider Using External BGP, page 2

• How to Connect to a Service Provider Using External BGP, page 14

• Configuration Examples for Connecting to a Service Provider Using External BGP, page 73

• Where to Go Next, page 84

• Additional References, page 85

• Feature Information for Connecting to a Service Provider Using External BGP, page 86

Finding Feature InformationYour software release may not support all the features documented in this module. For the latest featureinformation and caveats, see the release notes for your platform and software release. To find informationabout the features documented in this module, and to see a list of the releases in which each feature issupported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support.To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Americas Headquarters:Cisco Systems, Inc., 170 West Tasman Drive, San Jose, CA 95134-1706 USA

http://www.cisco.com/go/cfn

Prerequisites for Connecting to a Service Provider UsingExternal BGP

• Before connecting to a service provider you need to understand how to configure the basic BGPprocess and peers. See the "Cisco BGP Overview" and "Configuring a Basic BGP Network" modulesfor more details.

• The tasks and concepts in this chapter will help you configure BGP features that would be useful ifyou are connecting your network to a service provider. For each connection to the Internet, you musthave an assigned autonomous system number from the Internet Assigned Numbers Authority (IANA).

Restrictions for Connecting to a Service Provider UsingExternal BGP

• A router that runs Cisco IOS software can be configured to run only one BGP routing process and tobe a member of only one BGP autonomous system. However, a BGP routing process and autonomoussystem can support multiple address family configurations.

• Policy lists are not supported in versions of Cisco IOS software prior to Cisco IOS Release 12.0(22)Sand 12.2(15)T. Reloading a router that is running an older version of Cisco IOS software may causesome routing policy configurations to be lost.

Information About Connecting to a Service Provider UsingExternal BGP

• External BGP Peering, page 3

• BGP Autonomous System Number Formats, page 4

• BGP Attributes, page 6

• Multihoming, page 8

• MED Attribute, page 8

• Transit Versus Nontransit Traffic, page 9

• BGP Policy Configuration, page 9

• BGP Prefix-Based Outbound Route Filtering, page 10

• BGP Communities, page 10

• Extended Communities, page 11

• Extended Community Lists, page 12

• Administrative Distance, page 12

• BGP Route Map Policy Lists, page 12

• BGP Route Map with a Continue Clause, page 13

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External BGP PeeringBGP is an interdomain routing protocol designed to provide loop-free routing links between organizations.BGP is designed to run over a reliable transport protocol and it uses TCP (port 179) as the transportprotocol. The destination TCP port is assigned 179, and the local port is assigned a random port number.Cisco IOS software supports BGP version 4, which has been used by ISPs to help build the Internet. RFC1771 introduced and discussed a number of new BGP features to allow the protocol to scale for Internetuse.

External BGP peering sessions are configured to allow BGP peers from different autonomous systems toexchange routing updates. By design, a BGP routing process expects eBGP peers to be directly connected,for example, over a WAN connection. However, there are many real-world scenarios where this rule wouldprevent routing from occurring. Peering sessions for multihop neighbors are configured with the neighborebgp-multihop command. The figure below shows simple eBGP peering between three routers. Router Bpeers with Router A and Router E. In the figure below, the neighbor ebgp-multihop command could beused to establish peering between Router A and Router E although this is a very simple network design.BGP forwards information about the next hop in the network using the NEXT_HOP attribute, which is setto the IP address of the interface that advertises a route in an eBGP peering session by default. The sourceinterface can be a physical interface or a loopback interface.

Figure 1 BGP Peers in Different Autonomous Systems

Loopback interfaces are preferred for establishing eBGP peering sessions because loopback interfaces areless susceptible to interface flapping. Interfaces on networking devices can fail, and they can also be takenout of service for maintenance. When an interface is administratively brought up or down, due to failure ormaintenance, it is referred to as a flap. Loopback interfaces provide a stable source interface to ensure thatthe IP address assigned to the interface is always reachable as long as the IP routing protocols continue toadvertise the subnet assigned to the loopback interface. Loopback interfaces allow you to conserve addressspace by configuring a single address with /32 bit mask. Before a loopback interface is configured for aneBGP peering session, you must configure the neighbor update-source command and specify theloopback interface. With this configuration, the loopback interface becomes the source interface and its IPaddress is advertised as the next hop for routes that are advertised through this loopback. If loopbackinterfaces are used to connect single-hop eBGP peers, you must configure the neighbor disable-connected-check command before you can establish the eBGP peering session.

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Connecting to external networks enables traffic from your network to be forwarded to other networks andacross the Internet. Traffic will also be flowing into, and possibly through, your network. BGP containsvarious techniques to influence how the traffic flows into and out of your network, and to create BGPpolicies that filter the traffic, inbound and outbound. To influence the traffic flow, BGP uses certain BGPattributes that can be included in update messages or used by the BGP routing algorithm. BGP policies tofilter traffic also use some of the BGP attributes with route maps, access lists including AS-path accesslists, filter lists, policy lists, and distribute lists. Managing your external connections may involvemultihoming techniques where there is more than one connection to an ISP or connections to more than oneISP for backup or performance purposes. Tagging BGP routes with different community attributes acrossautonomous system or physical boundaries can prevent the need to configure long lists of individual permitor deny statements.

BGP Autonomous System Number FormatsPrior to January 2009, BGP autonomous system numbers that were allocated to companies were 2-octetnumbers in the range from 1 to 65535 as described in RFC 4271, A Border Gateway Protocol 4 (BGP-4).Due to increased demand for autonomous system numbers, the Internet Assigned Number Authority(IANA) will start in January 2009 to allocate four-octet autonomous system numbers in the range from65536 to 4294967295. RFC 5396, Textual Representation of Autonomous System (AS) Numbers ,documents three methods of representing autonomous system numbers. Cisco has implemented thefollowing two methods:

• Asplain--Decimal value notation where both 2-byte and 4-byte autonomous system numbers arerepresented by their decimal value. For example, 65526 is a 2-byte autonomous system number and234567 is a 4-byte autonomous system number.

• Asdot--Autonomous system dot notation where 2-byte autonomous system numbers are represented bytheir decimal value and 4-byte autonomous system numbers are represented by a dot notation. Forexample, 65526 is a 2-byte autonomous system number and 1.169031 is a 4-byte autonomous systemnumber (this is dot notation for the 234567 decimal number).

For details about the third method of representing autonomous system numbers, see RFC 5396.

Asdot Only Autonomous System Number Formatting

In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and later releases, the 4-octet (4-byte) autonomous systemnumbers are entered and displayed only in asdot notation, for example, 1.10 or 45000.64000. When usingregular expressions to match 4-byte autonomous system numbers the asdot format includes a period whichis a special character in regular expressions. A backslash must be entered before the period for example, 1\.14, to ensure the regular expression match does not fail. The table below shows the format in which 2-byteand 4-byte autonomous system numbers are configured, matched in regular expressions, and displayed inshow command output in Cisco IOS images where only asdot formatting is available.

Table 1 Asdot Only 4-Byte Autonomous System Number Format

Format Configuration Format Show Command Output andRegular Expression MatchFormat

asdot 2-byte: 1 to 65535 4-byte: 1.0 to65535.65535

2-byte: 1 to 65535 4-byte: 1.0 to65535.65535

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Asplain as Default Autonomous System Number Formatting

In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, and laterreleases, the Cisco implementation of 4-byte autonomous system numbers uses asplain as the defaultdisplay format for autonomous system numbers, but you can configure 4-byte autonomous system numbersin both the asplain and asdot format. In addition, the default format for matching 4-byte autonomoussystem numbers in regular expressions is asplain, so you must ensure that any regular expressions to match4-byte autonomous system numbers are written in the asplain format. If you want to change the defaultshow command output to display 4-byte autonomous system numbers in the asdot format, use the bgpasnotation dot command under router configuration mode. When the asdot format is enabled as thedefault, any regular expressions to match 4-byte autonomous system numbers must be written using theasdot format, or the regular expression match will fail. The tables below show that although you canconfigure 4-byte autonomous system numbers in either asplain or asdot format, only one format is used todisplay show command output and control 4-byte autonomous system number matching for regularexpressions, and the default is asplain format. To display 4-byte autonomous system numbers in showcommand output and to control matching for regular expressions in the asdot format, you must configurethe bgp asnotation dot command. After enabling the bgp asnotation dot command, a hard reset must beinitiated for all BGP sessions by entering the clear ip bgp * command.

Note If you are upgrading to an image that supports 4-byte autonomous system numbers, you can still use 2-byteautonomous system numbers. The show command output and regular expression match are not changedand remain in asplain (decimal value) format for 2-byte autonomous system numbers regardless of theformat configured for 4-byte autonomous system numbers.

Table 2 Default Asplain 4-Byte Autonomous System Number Format


asplain 2-byte: 1 to 65535 4-byte: 65536to 4294967295

2-byte: 1 to 65535 4-byte: 65536to 4294967295


2-byte: 1 to 65535 4-byte: 65536to 4294967295

Table 3 Asdot 4-Byte Autonomous System Number Format


asplain 2-byte: 1 to 65535 4-byte: 65536to 4294967295

2-byte: 1 to 65535 4-byte: 1.0 to65535.65535


2-byte: 1 to 65535 4-byte: 1.0 to65535.65535

Reserved and Private Autonomous System Numbers

In Cisco IOS Release 12.0(32)S12, 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1,12.4(24)T, and later releases, the Cisco implementation of BGP supports RFC 4893. RFC 4893 was

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developed to allow BGP to support a gradual transition from 2-byte autonomous system numbers to 4-byteautonomous system numbers. A new reserved (private) autonomous system number, 23456, was created byRFC 4893 and this number cannot be configured as an autonomous system number in the Cisco IOS CLI.

RFC 5398, Autonomous System (AS) Number Reservation for Documentation Use, describes new reservedautonomous system numbers for documentation purposes. Use of the reserved numbers allow configurationexamples to be accurately documented and avoids conflict with production networks if these configurationsare literally copied. The reserved numbers are documented in the IANA autonomous system numberregistry. Reserved 2-byte autonomous system numbers are in the contiguous block, 64496 to 64511 andreserved 4-byte autonomous system numbers are from 65536 to 65551 inclusive.

Private 2-byte autonomous system numbers are still valid in the range from 64512 to 65534 with 65535being reserved for special use. Private autonomous system numbers can be used for internal routingdomains but must be translated for traffic that is routed out to the Internet. BGP should not be configured toadvertise private autonomous system numbers to external networks. Cisco IOS software does not removeprivate autonomous system numbers from routing updates by default. We recommend that ISPs filterprivate autonomous system numbers.

Note Autonomous system number assignment for public and private networks is governed by the IANA. Forinformation about autonomous-system numbers, including reserved number assignment, or to apply toregister an autonomous system number, see the following URL: http://www.iana.org/.

BGP AttributesBGP selects a single path, by default, as the best path to a destination host or network. The best-pathselection algorithm analyzes path attributes to determine which route is installed as the best path in theBGP routing table. Each path carries various attributes that are used in BGP best-path analysis. Cisco IOSsoftware provides the ability to influence BGP path selection by altering these attributes via the command-line interface (CLI). BGP path selection can also be influenced through standard BGP policy configuration.

BGP uses the best-path selection algorithm to find a set of equally good routes. These routes are thepotential multipaths. In Cisco IOS Release 12.2(33)SRD and later releases, when there are more equallygood multipaths available than the maximum permitted number, then the oldest paths are selected asmultipaths.

BGP can include path attribute information in update messages. BGP attributes describe the characteristicof the route, and the software uses these attributes to help make decisions about which routes to advertise.Some of this attribute information can be configured at a BGP-speaking networking device. There are somemandatory attributes that are always included in the update message and some discretionary attributes. Thefollowing BGP attributes can be configured:

• AS-path• Community• Local_Pref• Multi_Exit_Discriminator (MED)• Next_Hop• Origin

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AS-path

This attribute contains a list or set of the autonomous system numbers through which routing informationhas passed. The BGP speaker adds its own autonomous system number to the list when it forwards theupdate message to external peers.

Community

BGP communities are used to group networking devices that share common properties, regardless ofnetwork, autonomous system, or any physical boundaries. In large networks applying a common routingpolicy through prefix lists or access lists requires individual peer statements on each networking device.Using the BGP community attribute BGP neighbors, with common routing policies, can implementinbound or outbound route filters based on the community tag rather than consult large lists of individualpermit or deny statements.

Local_Pref

Within an autonomous system, the Local_Pref attribute is included in all update messages between BGPpeers. If there are several paths to the same destination, the local preference attribute with the highest valueindicates the preferred outbound path from the local autonomous system. The highest ranking route isadvertised to internal peers. The Local_Pref value is not forwarded to external peers.

Multi_Exit_Discriminator

The MED attribute indicates (to an external peer) a preferred path into an autonomous system. If there aremultiple entry points into an autonomous system, the MED can be used to influence another autonomoussystem to choose one particular entry point. A metric is assigned where a lower MED metric is preferred bythe software over a higher MED metric. The MED metric is exchanged between autonomous systems, butafter a MED is forwarded into an autonomous system, the MED metric is reset to the default value of 0.When an update is sent to an internal BGP (iBGP) peer, the MED is passed along without any change,allowing all the peers in the same autonomous system to make a consistent path selection.

By default, a router will compare the MED attribute for paths only from BGP peers that reside in the sameautonomous system. The bgp always-compare-med command can be configured to allow the router tocompare metrics from peers in different autonomous systems.

Note The Internet Engineering Task Force (IETF) decision regarding BGP MED assigns a value of infinity to themissing MED, making the route that lacks the MED variable the least preferred. The default behavior ofBGP routers that run Cisco IOS software is to treat routes without the MED attribute as having a MED of 0,making the route that lacks the MED variable the most preferred. To configure the router to conform to theIETF standard, use the bgp bestpath med missing-as-worstrouter configuration command.

Next_Hop

The Next_Hop attribute identifies the next-hop IP address to be used as the BGP next hop to thedestination. The router makes a recursive lookup to find the BGP next hop in the routing table. In externalBGP (eBGP), the next hop is the IP address of the peer that sent the update. Internal BGP (iBGP) sets thenext-hop address to the IP address of the peer that advertised the prefix for routes that originate internally.When any routes to iBGP that are learned from eBGP are advertised, the Next_Hop attribute is unchanged.

A BGP next-hop IP address must be reachable in order for the router to use a BGP route. Reachabilityinformation is usually provided by the IGP, and changes in the IGP can influence the forwarding of thenext-hop address over a network backbone.

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Origin

This attribute indicates how the route was included in a BGP routing table. In Cisco IOS software, a routedefined using the BGP network command is given an origin code of Interior Gateway Protocol (IGP).Routes distributed from an Exterior Gateway Protocol (EGP) are coded with an origin of EGP, and routesredistributed from other protocols are defined as Incomplete. BGP decision policy for origin prefers IGPover EGP, and then EGP over Incomplete.

MultihomingMultihoming is defined as connecting an autonomous system with more than one service provider. If youhave any reliability issues with one service provider, then you have a backup connection. Performanceissues can also be addressed by multihoming because better paths to the destination network can beutilized.

Unless you are a service provider, you must plan your routing configuration carefully to avoid Internettraffic traveling through your autonomous system and consuming all your bandwidth. The figure belowshows that autonomous system 45000 is multihomed to autonomous system 40000 and autonomous system50000. Assuming autonomous system 45000 is not a service provider, then several techniques such as loadbalancing or some form of routing policy must be configured to allow traffic from autonomous system45000 to reach either autonomous system 40000 or autonomous system 50000 but not allow much, if any,transit traffic.

Figure 2 Multihoming Topology

MED AttributeConfiguring the MED attribute is another method that BGP can use to influence the choice of paths intoanother autonomous system. The MED attribute indicates (to an external peer) a preferred path into anautonomous system. If there are multiple entry points into an autonomous system, the MED can be used toinfluence another autonomous system to choose one particular entry point. A metric is assigned using routemaps where a lower MED metric is preferred by the software over a higher MED metric.

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Transit Versus Nontransit TrafficMost of the traffic within an autonomous system contains a source or destination IP address residing withinthe autonomous system, and this traffic is referred to as nontransit (or local) traffic. Other traffic is definedas transit traffic. As traffic across the Internet increases, controlling transit traffic becomes more important.

A service provider is considered to be a transit autonomous system and must provide connectivity to allother transit providers. In reality, few service providers actually have enough bandwidth to allow all transittraffic, and most service providers have to purchase such connectivity from Tier 1 service providers.

An autonomous system that does not usually allow transit traffic is called a stub autonomous system andwill link to the Internet through one service provider.

BGP Policy ConfigurationBGP policy configuration is used to control prefix processing by the BGP routing process and to filterroutes from inbound and outbound advertisements. Prefix processing can be controlled by adjusting BGPtimers, altering how BGP handles path attributes, limiting the number of prefixes that the routing processwill accept, and configuring BGP prefix dampening. Prefixes in inbound and outbound advertisements arefiltered using route maps, filter lists, IP prefix lists, autonomous-system-path access lists, IP policy lists,and distribute lists. The table below shows the processing order of BGP policy filters.

Table 4 BGP Policy Processing Order

Inbound Outbound

Route map Distribute list

Filter list, AS-path access list, or IP policy IP prefix list

IP prefix list Filter list, AS-path access list, or IP policy

Distribute list Route map

Note In Cisco IOS Releases 12.0(22)S, 12.2(15)T, 12.2(18)S, and later releases, the maximum number ofautonomous system access lists that can be configured with the ip as-path access-list command isincreased from 199 to 500.

Whenever there is a change in the routing policy due to a configuration change, BGP peering sessions mustbe reset using the clear ip bgp command. Cisco IOS software supports the following three mechanisms toreset BGP peering sessions:

• Hard reset--A hard reset tears down the specified peering sessions, including the TCP connection, anddeletes routes coming from the specified peer.

• Soft reset--A soft reset uses stored prefix information to reconfigure and activate BGP routing tableswithout tearing down existing peering sessions. Soft reset uses stored update information, at the cost ofadditional memory for storing the updates, to allow you to apply a new BGP policy without disruptingthe network. Soft reset can be configured for inbound or outbound sessions.

• Dynamic inbound soft reset--The route refresh capability, as defined in RFC 2918, allows the localrouter to reset inbound routing tables dynamically by exchanging route refresh requests to supportingpeers. The route refresh capability does not store update information locally for nondisruptive policychanges. It instead relies on dynamic exchange with supporting peers. Route refresh must first be

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advertised through BGP capability negotiation between peers. All BGP routers must support the routerefresh capability.

To determine if a BGP router supports this capability, use the show ip bgp neighborscommand. Thefollowing message is displayed in the output when the router supports the route refresh capability:

Received route refresh capability from peer.

BGP Prefix-Based Outbound Route FilteringBGP prefix-based outbound route filtering uses the BGP ORF send and receive capabilities to minimize thenumber of BGP updates that are sent between BGP peers. Configuring BGP ORF can help reduce theamount of system resources required for generating and processing routing updates by filtering outunwanted routing updates at the source. For example, BGP ORF can be used to reduce the amount ofprocessing required on a router that is not accepting full routes from a service provider network.

The BGP prefix-based outbound route filtering is enabled through the advertisement of ORF capabilities topeer routers. The advertisement of the ORF capability indicates that a BGP peer will accept a prefix listfrom a neighbor and apply the prefix list to locally configured ORFs (if any exist). When this capability isenabled, the BGP speaker can install the inbound prefix list filter to the remote peer as an outbound filter,which reduces unwanted routing updates.

The BGP prefix-based outbound route filtering can be configured with send or receive ORF capabilities.The local peer advertises the ORF capability in send mode. The remote peer receives the ORF capability inreceive mode and applies the filter as an outbound policy. The local and remote peers exchange updates tomaintain the ORF on each router. Updates are exchanged between peer routers by address familydepending on the ORF prefix list capability that is advertised. The remote peer starts sending updates to thelocal peer after a route refresh has been requested with the clear ip bgp in prefix-filtercommand or afteran ORF prefix list with immediate status is processed. The BGP peer will continue to apply the inboundprefix list to received updates after the local peer pushes the inbound prefix list to the remote peer.

BGP CommunitiesBGP communities are used to group routes (also referred to as color routes) that share common properties,regardless of network, autonomous system, or any physical boundaries. In large networks applying acommon routing policy through prefix-lists or access-lists requires individual peer statements on eachnetworking device. Using the BGP community attribute BGP speakers, with common routing policies, canimplement inbound or outbound route filters based on the community tag rather than consult large lists ofindividual permit or deny statements.

Standard community lists are used to configure well-known communities and specific community numbers.Expanded community lists are used to filter communities using a regular expression. Regular expressionsare used to configure patterns to match community attributes.

The community attribute is optional, which means that it will not be passed on by networking devices thatdo not understand communities. Networking devices that understand communities must be configured tohandle the communities or they will be discarded.

There are four predefined communities:

• no-export--Do not advertise to external BGP peers.• no-advertise--Do not advertise this route to any peer.• internet--Advertise this route to the Internet community; all BGP-speaking networking devices belong

to it.• local-as--Do not send outside the local autonomous system.

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In Cisco IOS Release 12.2(8)T, BGP named community lists were introduced. BGP named community listsallow meaningful names to be assigned to community lists with no limit on the number of community liststhat can be configured. A named community list can be configured with regular expressions and withnumbered community lists. All the rules of numbered communities apply to named community lists exceptthat there is no limitation on the number of named community lists that can be configured.

Note Both standard and expanded community lists have a limitation of 100 community groups that can beconfigured within each type of list. A named community list does not have this limitation.

Extended CommunitiesExtended community attributes are used to configure, filter, and identify routes for virtual routing andforwarding (VRF) instances and Multiprotocol Label Switching (MPLS) Virtual Private Networks (VPNs).All of the standard rules of access lists apply to the configuration of extended community lists. Regularexpressions are supported by the expanded range of extended community list numbers. All regularexpression configuration options are supported. The route target (RT) and site of origin (SoO) extendedcommunity attributes are supported by the standard range of extended community lists.

Route Target Extended Community Attribute

The RT extended community attribute is configured with the rt keyword of the ip extcommunity-listcommand. This attribute is used to identify a set of sites and VRFs that may receive routes that aretagged with the configured route target. Configuring the route target extended community attribute with aroute allows that route to be placed in the per-site forwarding tables that are used for routing traffic that isreceived from corresponding sites.

Site of Origin Extended Community Attribute

The SoO extended community attribute is configured with the soo keyword of the ip extcommunity-listcommand. This attribute uniquely identifies the site from which the provider edge (PE) router learnedthe route. All routes learned from a particular site must be assigned the same SoO extended communityattribute, regardless if a site is connected to a single PE router or multiple PE routers. Configuring thisattribute prevents routing loops from occurring when a site is multihomed. The SoO extended communityattribute is configured on the interface and is propagated into BGP through redistribution. The SoOextended community attribute can be applied to routes that are learned from VRFs. The SoO extendedcommunity attribute should not be configured for stub sites or sites that are not multihomed.

IP Extended Community-List Configuration Mode

Named and numbered extended community lists can be configured in IP extended community-listconfiguration mode. The IP extended community-list configuration mode supports all of the functions thatare available in global configuration mode. In addition, the following operations can be performed:

• Configure sequence numbers for extended community list entries.• Resequence existing sequence numbers for extended community list entries.• Configure an extended community list to use default values.

Default Sequence Numbering

Extended community list entries start with the number 10 and increment by 10 for each subsequent entrywhen no sequence number is specified, when default behavior is configured, and when an extended

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community list is resequenced without specifying the first entry number or the increment range forsubsequent entries.

Resequencing Extended Community Lists

Extended community-list entries are sequenced and resequenced on a per-extended community list basis.The resequence command can be used without any arguments to set all entries in a list to default sequencenumbering. The resequence command also allows the sequence number of the first entry and incrementrange to be set for each subsequent entry. The range of configurable sequence numbers is from 1 to2147483647.

Extended Community ListsExtended community attributes are used to configure, filter, and identify routes for VRF instances andMPLS VPNs. The ip extcommunity-listcommand is used to configure named or numbered extendedcommunity lists. All of the standard rules of access lists apply to the configuration of extended communitylists. Regular expressions are supported by the expanded range of extended community list numbers.

Administrative DistanceAdministrative distance is a measure of the preference of different routing protocols. BGP has a distancebgp command that allows you to set different administrative distances for three route types: external,internal, and local. BGP, like other protocols, prefers the route with the lowest administrative distance.

BGP Route Map Policy ListsBGP route map policy lists allow a network operator to group route map match clauses into named listscalled policy lists. A policy list functions like a macro. When a policy list is referenced in a route map, allof the match clauses are evaluated and processed as if they had been configured directly in the route map.This enhancement simplifies the configuration of BGP routing policy in medium-size and large networksbecause a network operator can preconfigure policy lists with groups of match clauses and then referencethese policy lists within different route maps. The network operator no longer needs to manuallyreconfigure each recurring group of match clauses that occur in multiple route map entries.

A policy lists functions like a macro when it is configured in a route map and has the following capabilitiesand characteristics:

• When a policy list is referenced within a route map, all the match statements within the policy list areevaluated and processed.

• Two or more policy lists can be configured with a route map. Policy lists can be configured within aroute map to be evaluated with AND or OR semantics.

• Policy lists can coexist with any other preexisting match and set statements that are configured withinthe same route map but outside of the policy lists.

• When multiple policy lists perform matching within a route map entry, all policy lists match on theincoming attribute only.

Policy lists support only match clauses and do not support set clauses. Policy lists can be configured for allapplications of route maps, including redistribution, and can also coexist, within the same route map entry,with match and set clauses that are configured separately from the policy lists.

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Note Policy lists are supported only by BGP and are not supported by other IP routing protocols.

BGP Route Map with a Continue ClauseIn Cisco IOS Release 12.3(2)T, 12.0(24)S, 12.2(33)SRB, and later releases, the continue clause wasintroduced into BGP route map configuration. The continue clause allows for more programmable policyconfiguration and route filtering and introduced the capability to execute additional entries in a route mapafter an entry is executed with successful match and set clauses. Continue clauses allow the networkoperator to configure and organize more modular policy definitions so that specific policy configurationsneed not be repeated within the same route map. Before the continue clause was introduced, route mapconfiguration was linear and did not allow any control over the flow of a route map.

In Cisco IOS Release 12.0(31)S, 12.2(33)SB, 12.2(33)SRB, 12.2(33)SXI, 12.4(4)T, and later releases,support for continue clauses for outbound route maps was introduced.

• Route Map Operation Without Continue Clauses, page 13• Route Map Operation with Continue Clauses, page 13• Match Operations with Continue Clauses, page 13• Set Operations with Continue Clauses, page 14

Route Map Operation Without Continue ClausesA route map evaluates match clauses until a successful match occurs. After the match occurs, the route mapstops evaluating match clauses and starts executing set clauses, in the order in which they were configured.If a successful match does not occur, the route map "falls through" and evaluates the next sequence numberof the route map until all configured route map entries have been evaluated or a successful match occurs.Each route map sequence is tagged with a sequence number to identify the entry. Route map entries areevaluated in order starting with the lowest sequence number and ending with the highest sequence number.If the route map contains only set clauses, the set clauses will be executed automatically, and the route mapwill not evaluate any other route map entries.

Route Map Operation with Continue ClausesWhen a continue clause is configured, the route map will continue to evaluate and execute match clauses inthe specified route map entry after a successful match occurs. The continue clause can be configured to goto (or jump to) a specific route map entry by specifying the sequence number, or if a sequence number isnot specified, the continue clause will go to the next sequence number. This behavior is called an "impliedcontinue." If a match clause exists, the continue clause is executed only if a match occurs. If no successfulmatches occur, the continue clause is ignored.

Match Operations with Continue ClausesIf a match clause does not exist in the route map entry but a continue clause does, the continue clause willbe automatically executed and go to the specified route map entry. If a match clause exists in a route mapentry, the continue clause is executed only when a successful match occurs. When a successful matchoccurs and a continue clause exists, the route map executes the set clauses and then goes to the specifiedroute map entry. If the next route map entry contains a continue clause, the route map will execute thecontinue clause if a successful match occurs. If a continue clause does not exist in the next route map entry,the route map will be evaluated normally. If a continue clause exists in the next route map entry but a

BGP Route Map with a Continue ClauseRoute Map Operation Without Continue Clauses

13

match does not occur, the route map will not continue and will "fall through" to the next sequence numberif one exists.

Set Operations with Continue ClausesSet clauses are saved during the match clause evaluation process and executed after the route-mapevaluation is completed. The set clauses are evaluated and executed in the order in which they wereconfigured. Set clauses are executed only after a successful match occurs, unless the route map does notcontain a match clause. The continue statement proceeds to the specified route map entry only afterconfigured set actions are performed. If a set action occurs in the first route map and then the same setaction occurs again, with a different value, in a subsequent route map entry, the last set action may overrideany previous set actions that were configured with the same set command unless the set command permitsmore than one value. For example, the set as-path prepend command permits more than one autonomoussystem number to be configured.

Note A continue clause can be executed, without a successful match, if a route map entry does not contain amatch clause.

Note Route maps have a linear behavior and not a nested behavior. Once a route is matched in a route mappermit entry with a continue command clause, it will not be processed by the implicit deny at the end of theroute-map. For an example, see "Filtering Traffic Using Continue Clauses in a BGP Route Map Examples".

• Restrictions, page 14

Restrictions

• Continue clauses for outbound route maps are supported only in Cisco IOS Release 12.0(31)S,12.2(33)SB, 12.2(33)SRB, 12.2(33)SXI, 12.4(4)T, and later releases.

• Continue clauses can go only to a higher route map entry (a route map entry with a higher sequencenumber) and cannot go to a lower route map entry.

How to Connect to a Service Provider Using External BGP• Influencing Inbound Path Selection, page 14

• Influencing Outbound Path Selection, page 23

• Configuring BGP Peering with ISPs, page 29

• Configuring BGP Policies, page 46

Influencing Inbound Path SelectionBGP can be used to influence the choice of paths in another autonomous system. There may be severalreasons for wanting BGP to choose a path that is not the obvious best route, for example, to avoid sometypes of transit traffic passing through an autonomous system or perhaps to avoid a very slow or congestedlink. BGP can influence inbound path selection using one of the following BGP attributes:

Influencing Inbound Path Selection Set Operations with Continue Clauses

14

• AS-path• MED

Perform one of the following tasks to influence inbound path selection:

• Influencing Inbound Path Selection by Modifying the AS-path Attribute, page 15

• Influencing Inbound Path Selection by Setting the MED Attribute, page 19

Influencing Inbound Path Selection by Modifying the AS-path AttributePerform this task to influence the inbound path selection for traffic destined for the 172.17.1.0 network bymodifying the AS-path attribute. The configuration is performed at Router A in the figure below. For aconfiguration example of this task using 4-byte autonomous system numbers in asplain format, see Influencing Inbound Path Selection by Modifying the AS-path Attribute Using 4-Byte AS NumbersExample, page 74.

One of the methods that BGP can use to influence the choice of paths in another autonomous system is tomodify the AS-path attribute. For example, in the figure below, Router A advertises its own network,172.17.1.0, to its BGP peers in autonomous system 45000 and autonomous system 60000. When therouting information is propagated to autonomous system 50000, the routers in autonomous system 50000have network reachability information about network 172.17.1.0 from two different routes. The first routeis from autonomous system 45000 with an AS-path consisting of 45000, 40000, the second route is throughautonomous system 55000 with an AS-path of 55000, 60000, 40000. If all other BGP attribute values arethe same, Router C in autonomous system 50000 would choose the route through autonomous system45000 for traffic destined for network 172.17.1.0 because it is the shortest route in terms of autonomoussystems traversed.

Autonomous system 40000 now receives all traffic from autonomous system 50000 for the 172.17.1.0network through autonomous system 45000. If, however, the link between autonomous system 45000 andautonomous system 40000 is a really slow and congested link, the set as-path prependcommand can beused at Router A to influence inbound path selection for the 172.17.1.0 network by making the routethrough autonomous system 45000 appear to be longer than the path through autonomous system 60000.The configuration is done at Router A in the figure below by applying a route map to the outbound BGPupdates to Router B. Using the set as-path prependcommand, all the outbound BGP updates from RouterA to Router B will have their AS-path attribute modified to add the local autonomous system number40000 twice. After the configuration, autonomous system 50000 receives updates about the 172.17.1.0network through autonomous system 45000. The new AS-path is 45000, 40000, 40000, and 40000, whichis now longer than the AS-path from autonomous system 55000 (unchanged at a value of 55000, 60000,

Influencing Inbound Path SelectionInfluencing Inbound Path Selection by Modifying the AS-path Attribute

15

40000). Networking devices in autonomous system 50000 will now prefer the route through autonomoussystem 55000 to forward packets with a destination address in the 172.17.1.0 network.

Figure 3 Network Topology for Modifying the AS-path Attribute

SUMMARY STEPS

1. enable

2. configure terminal

3. router bgp autonomous-system-number

4. neighbor {ip-address | peer-group-name} remote-as autonomous-system-number

5. address-family ipv4 [unicast | multicast| vrf vrf-name]

6. network network-number [mask network-mask][route-map route-map-name]

7. neighbor {ip-address | peer-group-name} route-map map-name{in | out}

8. neighbor {ip-address| peer-group-name} activate

9. exit-address-family

10. exit

11. route-map map-name [permit| deny][sequence-number]

12. set as-path {tag | prepend as-path-string}

13. end

14. show running-config

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DETAILED STEPS

Command or Action Purpose

Step 1 enable

Example:

Router> enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Step 2 configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 router bgp autonomous-system-number

Example:

Router(config)# router bgp 40000

Enters router configuration mode for the specified routing process.

Step 4 neighbor {ip-address | peer-group-name}remote-as autonomous-system-number

Example:

Router(config-router)# neighbor 192.168.1.2 remote-as 45000

Adds the IP address or peer group name of the neighbor in thespecified autonomous system to the IPv4 multiprotocol BGP neighbortable of the local router.

• In this example, the BGP peer on Router B at 192.168.1.2 isadded to the IPv4 multiprotocol BGP neighbor table and willreceive BGP updates.

Step 5 address-family ipv4 [unicast | multicast| vrfvrf-name]

Example:

Router(config-router)# address-family ipv4 unicast

Specifies the IPv4 address family and enters address familyconfiguration mode.

• The unicast keyword specifies the IPv4 unicast address family.By default, the router is placed in address family configurationmode for the IPv4 unicast address family if the unicast keywordis not specified with the address-family ipv4 command.

• The multicast keyword specifies IPv4 multicast address prefixes.• The vrf keyword and vrf-name argument specify the name of the

VRF instance to associate with subsequent IPv4 address familyconfiguration mode commands.

Step 6 network network-number [mask network-mask][route-map route-map-name]

Example:

Router(config-router-af)# network 172.17.1.0 mask 255.255.255.0

Specifies a network as local to this autonomous system and adds it tothe BGP routing table.

• For exterior protocols the network command controls whichnetworks are advertised. Interior protocols use the networkcommand to determine where to send updates.

Influencing Inbound Path SelectionInfluencing Inbound Path Selection by Modifying the AS-path Attribute

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Step 7 neighbor {ip-address | peer-group-name}route-map map-name{in | out}

Example:

Router(config-router-af)# neighbor 192.168.1.2 route-map PREPEND out

Applies a route map to incoming or outgoing routes.

• In this example, the route map named PREPEND is applied tooutbound routes to Router B.

Step 8 neighbor {ip-address| peer-group-name}activate

Example:

Router(config-router-af)# neighbor 192.168.1.2 activate

Enables address exchange for address family IPv4 unicast for the BGPneighbor at 192.168.1.2 on Router B.

Step 9 exit-address-family

Example:

Router(config-router-af)# exit

Exits address family configuration mode and enters routerconfiguration mode.

Step 10 exit

Example:

Router(config-router)# exit

Exits router configuration mode and enters global configuration mode.

Step 11 route-map map-name [permit| deny][sequence-number]

Example:

Router(config)# route-map PREPEND permit 10

Configures a route map and enters route map configuration mode.

• In this example, a route map named PREPEND is created with apermit clause.

Step 12 set as-path {tag | prepend as-path-string}

Example:

Router(config-route-map)# set as-path prepend 40000 40000

Modifies an autonomous system path for BGP routes.

• Use the prepend keyword to "prepend" an arbitrary autonomoussystem path string to BGP routes. Usually the local autonomoussystem number is prepended multiple times, increasing theautonomous system path length.

• In this example, two additional autonomous system entries areadded to the autonomous system path for outbound routes toRouter B.

Influencing Inbound Path Selection Influencing Inbound Path Selection by Modifying the AS-path Attribute

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Step 13 end

Example:

Router(config-route-map)# end

Exits route map configuration mode and returns to privileged EXECmode.

Step 14 show running-config

Example:

Router# show running-config

Displays the running configuration file.

Examples

Router A

The following partial output of the show running-config command shows the configuration from this task.

Router# show running-config...router bgp 40000 neighbor 192.168.1.2 remote-as 45000 ! address-family ipv4 neighbor 192.168.1.2 activate neighbor 192.168.1.2 route-map PREPEND out no auto-summary no synchronization network 172.17.1.0 mask 255.255.255.0 exit-address-family!route-map PREPEND permit 10 set as-path prepend 40000 40000...

Influencing Inbound Path Selection by Setting the MED AttributeOne of the methods that BGP can use to influence the choice of paths into another autonomous system is toset the MED attribute. The MED attribute indicates (to an external peer) a preferred path to an autonomoussystem. If there are multiple entry points to an autonomous system, the MED can be used to influenceanother autonomous system to choose one particular entry point. A metric is assigned using route mapswhere a lower MED metric is preferred by the software over a higher MED metric.

Perform this task to influence inbound path selection by setting the MED metric attribute. Theconfiguration is performed at Router B and Router D in the figure below. Router B advertises the network172.16.1.0. to its BGP peer, Router E in autonomous system 50000. Using a simple route map Router Bsets the MED metric to 50 for outbound updates. The task is repeated at Router D but the MED metric isset to 120. When Router E receives the updates from both Router B and Router D the MED metric is storedin the BGP routing table. Before forwarding packets to network 172.16.1.0, Router E compares theattributes from peers in the same autonomous system (both Router B and Router D are in autonomous

Influencing Inbound Path SelectionInfluencing Inbound Path Selection by Setting the MED Attribute

19

system 45000). The MED metric for Router B is less than the MED for Router D, so Router E will forwardthe packets through Router B.

Figure 4 Network Topology for Setting the MED Attribute

Use the bgp always-compare-med command to compare MED attributes from peers in other autonomoussystems.

SUMMARY STEPS

1. enable







8. exit

9. exit


11. set metric value

12. end

13. Repeat Step 1 through Step 12 at Router D.

14. show ip bgp [network] [network-mask]

Influencing Inbound Path Selection Influencing Inbound Path Selection by Setting the MED Attribute

20

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:




Example:




Example:


Adds the IP address or peer group name of the neighbor in thespecified autonomous system to the IPv4 multiprotocol BGPneighbor table of the local router.

Step 5 address-family ipv4 [unicast | multicast| vrf vrf-name]

Example:




• The multicast keyword specifies IPv4 multicast addressprefixes.

• The vrf keyword and vrf-name argument specify the name ofthe VRF instance to associate with subsequent IPv4 addressfamily configuration mode commands.

Influencing Inbound Path SelectionInfluencing Inbound Path Selection by Setting the MED Attribute

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Example:




Step 7 neighbor {ip-address | peer-group-name} route-map map-name{in | out}

Example:

Router(config-router-af)# neighbor 192.168.3.2 route-map MED out


• In this example, the route map named MED is applied tooutbound routes to the BGP peer at Router E.

Step 8 exit

Example:



Step 9 exit

Example:


Exits router configuration mode and enters global configurationmode.


Example:

Router(config)# route-map MED permit 10


• In this example, a route map named MED is created.

Step 11 set metric value

Example:

Router(config-route-map)# set metric 50

Sets the MED metric value.

Step 12 end

Example:


Exits route map configuration mode and enters privileged EXECmode.

Influencing Inbound Path Selection Influencing Inbound Path Selection by Setting the MED Attribute

22


Step 13 Repeat Step 1 through Step 12 at Router D. --

Step 14 show ip bgp [network] [network-mask]

Example:

Router# show ip bgp 172.17.1.0 255.255.255.0

(Optional) Displays the entries in the BGP routing table.

• Use this command at Router E in the figure above when bothRouter B and Router D have configured the MED attribute.

• Only the syntax applicable to this task is used in this example.For more details, see the Cisco IOS IP Routing: BGP CommandReference.

Examples

The following output is from Router E in the figure above after this task has been performed at both RouterB and Router D. Note the metric (MED) values for the two routes to network 172.16.1.0. The peer192.168.2.1 at Router D has a metric of 120 for the path to network 172.16.1.0 whereas the peer192.168.3.1 at Router B has a metric of 50. The entry for the peer 192.168.3.1 at Router B has the wordbest at the end of the entry to show that Router E will choose to send packets destined for network172.16.1.0 via Router B because the MED metric is lower.

Router# show ip bgp 172.16.1.0BGP routing table entry for 172.16.1.0/24, version 10Paths: (2 available, best #2, table Default-IP-Routing-Table) Advertised to update-groups: 1 45000 192.168.2.1 from 192.168.2.1 (192.168.2.1) Origin IGP, metric 120, localpref 100, valid, external 45000 192.168.3.1 from 192.168.3.1 (172.17.1.99) Origin IGP, metric 50, localpref 100, valid, external, best

Influencing Outbound Path SelectionBGP can be used to influence the choice of paths for outbound traffic from the local autonomous system.This section contains two methods that BGP can use to influence outbound path selection:

• Using the Local_Pref attribute• Using the BGP outbound route filter (ORF) capability

Perform one of the following tasks to influence outbound path selection:

• Influencing Outbound Path Selection Using the Local_Pref Attribute, page 23• Filtering Outbound BGP Route Prefixes, page 26

Influencing Outbound Path Selection Using the Local_Pref AttributeOne of the methods to influence outbound path selection is to use the BGP Local-Pref attribute. Performthis task using the local preference attribute to influence outbound path selection. If there are several pathsto the same destination the local preference attribute with the highest value indicates the preferred path.

Refer to the figure below for the network topology used in this task. Both Router B and Router C areconfigured. autonomous system 45000 receives updates for network 192.168.3.0 via autonomous system40000 and autonomous system 50000. Router B is configured to set the local preference value to 150 for allupdates to autonomous system 40000. Router C is configured to set the local preference value for all

Influencing Outbound Path SelectionInfluencing Outbound Path Selection Using the Local_Pref Attribute

23

updates to autonomous system 50000 to 200. After the configuration, local preference information isexchanged within autonomous system 45000. Router B and Router C now see that updates for network192.168.3.0 have a higher preference value from autonomous system 50000 so all traffic in autonomoussystem 45000 with a destination network of 192.168.3.0 is sent out via Router C.

Figure 5 Network Topology for Outbound Path Selection

SUMMARY STEPS

1. enable



4. neighbor {ip-address| peer-group-name} remote-as autonomous-system-number

5. bgp default local-preference value



8. neighbor {ip-address| peer-group-name} activate

9. end

10. Repeat Step 1 through Step 9 at Router C but change the IP address of the peer, the autonomous systemnumber, and set the local preference value to 200.


Influencing Outbound Path Selection Influencing Outbound Path Selection Using the Local_Pref Attribute

24

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:




Example:



Step 4 neighbor {ip-address| peer-group-name}remote-as autonomous-system-number

Example:



Step 5 bgp default local-preference value

Example:

Router(config-router)# bgp default local-preference 150

Changes the default local preference value.

• In this example, the local preference is changed to 150 for allupdates from autonomous system 40000 to autonomous system45000.

• By default, the local preference value is 100.


Example:






Influencing Outbound Path SelectionInfluencing Outbound Path Selection Using the Local_Pref Attribute

25



Example:




Step 8 neighbor {ip-address| peer-group-name}activate

Example:

Router(config-router-af)# neighbor 192.168.1.2 activate


Step 9 end

Example:

Router(config-router-af)# end


Step 10 Repeat Step 1 through Step 9 at Router C butchange the IP address of the peer, theautonomous system number, and set the localpreference value to 200.

--


Example:


Displays the entries in the BGP routing table.

• Enter this command at both Router B and Router C and note theLocal_Pref value. The route with the highest preference valuewill be the preferred route to network 192.168.3.0.

Note Only the syntax applicable to this task is used in this example.For more details, see the Cisco IOS IP Routing: BGP CommandReference.

Filtering Outbound BGP Route PrefixesPerform this task to use BGP prefix-based outbound route filtering to influence outbound path selection.

BGP peering sessions must be established, and BGP ORF capabilities must be enabled on eachparticipating router before prefix-based ORF announcements can be received.

Influencing Outbound Path Selection Filtering Outbound BGP Route Prefixes

26

Note• BGP prefix-based outbound route filtering does not support multicast.• IP addresses that are used for outbound route filtering must be defined in an IP prefix list. BGP

distribute lists and IP access lists are not supported.• Outbound route filtering is configured on only a per-address family basis and cannot be configured

under the general session or BGP routing process.• Outbound route filtering is configured for external peering sessions only.

>

SUMMARY STEPS

1. enable


3. ip prefix-list list-name [seq seq-value] {deny network / length | permit network / length}[ge ge-value][le le-value]



6. neighbor ip-address ebgp-multihop [hop-count]


8. neighbor ip-address capability orf prefix-list [send | receive | both]

9. neighbor {ip-address| peer-group-name} prefix-list prefix-list-name{in | out}

10. end

11. clear ip bgp {ip-address | *} in prefix-filter

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:



Influencing Outbound Path SelectionFiltering Outbound BGP Route Prefixes

27


Step 3 ip prefix-list list-name [seq seq-value]{deny network / length | permit network /length}[ge ge-value] [le le-value]

Example:

Router(config)# ip prefix-list FILTER seq 10 permit 192.168.1.0/24

Creates a prefix list for prefix-based outbound route filtering.

• Outbound route filtering supports prefix length matching, wildcard-based prefix matching, and exact address prefix matching on a peraddress-family basis.

• The prefix list is created to define the outbound route filter. The filtermust be created when the outbound route filtering capability isconfigured to be advertised in send mode or both mode. It is notrequired when a peer is configured to advertise receive mode only.

• The example creates a prefix list named FILTER that defines the192.168.1.0/24 subnet for outbound route filtering.


Example:


Enters router configuration mode, and creates a BGP routing process.


Example:


Establishes peering with the specified neighbor or peer group. BGPpeering must be established before ORF capabilities can be exchanged.

• The example establishes peering with the 10.1.1.1 neighbor.

Step 6 neighbor ip-address ebgp-multihop [hop-count]

Example:

Router(config-router)# neighbor 10.1.1.1 ebgp-multihop

Accepts or initiates BGP connections to external peers residing onnetworks that are not directly connected.

Step 7 address-family ipv4 [unicast | multicast|vrf vrf-name]

Example:


Specifies the IPv4 address family and enters address family configurationmode.

• The unicast keyword specifies the IPv4 unicast address family. Bydefault, the router is placed in address family configuration mode forthe IPv4 unicast address family if the unicast keyword is notspecified with the address-family ipv4 command.



Note Outbound route filtering is configured on a per-address familybasis.

Influencing Outbound Path Selection Filtering Outbound BGP Route Prefixes

28


Step 8 neighbor ip-address capability orf prefix-list [send | receive | both]

Example:

Router(config-router-af)# neighbor 10.1.1.1 capability orf prefix-list both

Enables the ORF capability on the local router, and enables ORFcapability advertisement to the BGP peer specified with the ip-addressargument.

• The send keyword configures a router to advertise ORF sendcapabilities.

• The receive keyword configures a router to advertise ORF receivecapabilities.

• The both keyword configures a router to advertise send and receivecapabilities.

• The remote peer must be configured to either send or receive ORFcapabilities before outbound route filtering is enabled.

• The example configures the router to advertise send and receivecapabilities to the 10.1.1.1 neighbor.

Step 9 neighbor {ip-address| peer-group-name}prefix-list prefix-list-name{in | out}

Example:

Router(config-router-af)# neighbor 10.1.1.1 prefix-list FILTER in

Applies an inbound prefix-list filter to prevent distribution of BGPneighbor information.

• In this example, the prefix list named FILTER is applied to incomingadvertisements from the 10.1.1.1 neighbor, which preventsdistribution of the 192.168.1.0/24 subnet.

Step 10 end

Example:


Exits address family configuration mode, and enters privileged EXECmode.

Step 11 clear ip bgp {ip-address | *} in prefix-filter

Example:

Router# clear ip bgp 10.1.1.1 in prefix-filter

Clears BGP outbound route filters and initiates an inbound soft reset.

• A single neighbor or all neighbors can be specified.

Note The inbound soft refresh must be initiated with the clear ip bgpcommand in order for this feature to function.

Configuring BGP Peering with ISPsBGP was developed as an interdomain routing protocol and connecting to ISPs is one of the main functionsof BGP. Depending on the size of your network and the purpose of your business, there are many differentways to connect to your ISP. Multihoming to one or more ISPs provides redundancy in case an externallink to an ISP fails. This section introduces some optional tasks that can be used to connect to a serviceprovider using multihoming techniques. Smaller companies may use just one ISP but require a backuproute to the ISP. Larger companies may have access to two ISPs, using one of the connections as a backup,or may need to configure a transit autonomous system.

Perform one of the following optional tasks to connect to one or more ISPs:

• Configuring Multihoming with Two ISPs, page 30

Configuring BGP Peering with ISPsFiltering Outbound BGP Route Prefixes

29

• Multihoming with a Single ISP, page 34

• Configuring Multihoming to Receive the Full Internet Routing Table, page 42

Configuring Multihoming with Two ISPsPerform this task to configure your network to access two ISPs. where one ISP is the preferred route andthe second ISP is a backup route. In the figure below Router B in autonomous system 45000 has BGP peersin two ISPs, autonomous system 40000 and autonomous system 50000. Using this task, Router B will beconfigured to prefer the route to the BGP peer at Router A in autonomous system 40000.

All routes learned from this neighbor will have an assigned weight. The route with the highest weight willbe chosen as the preferred route when multiple routes are available to a particular network.

Note The weights assigned with the set weight route-map configuration command override the weights assignedusing the neighbor weight command.

Figure 6 Multihoming with Two ISPs

Configuring BGP Peering with ISPs Configuring Multihoming with Two ISPs

30

SUMMARY STEPS

1. enable





6. network network-number [mask network-mask]

7. neighbor {ip-address| peer-group-name} weight number

8. exit



11. neighbor {ip-address| peer-group-name} weight number

12. end

13. clear ip bgp {*| ip-address| peer-group-name} [soft [in | out]]


DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:




Example:




Example:


Adds the IP address or peer group name of the neighbor in the specifiedautonomous system to the IPv4 multiprotocol BGP neighbor table of thelocal router.

Configuring BGP Peering with ISPsConfiguring Multihoming with Two ISPs

31



Example:



• The unicast keyword specifies the IPv4 unicast address family. Bydefault, the router is placed in configuration mode for the IPv4unicast address family if the unicast keyword is not specified withthe address-family ipv4 command.



Step 6 network network-number [mask network-mask]

Example:


Specifies a network as local to this autonomous system and adds it to theBGP routing table.


Step 7 neighbor {ip-address| peer-group-name}weight number

Example:

Router(config-router-af)# neighbor 192.168.1.2 weight 150

Assigns a weight to a BGP peer connection.

• In this example, the weight attribute for routes received from theBGP peer 192.168.1.2 is set to 150.

Step 8 exit

Example:


Exits address family configuration mode and enters router configurationmode.


Example:



Configuring BGP Peering with ISPs Configuring Multihoming with Two ISPs

32



Example:



• The unicast keyword specifies the IPv4 unicast address family. Bydefault, the router is placed in configuration mode for the IPv4unicast address family if the unicast keyword is not specified withthe address-family ipv4 command.

• The multicast keyword specifies IPv4 multicast address prefixes.

The vrf keyword and vrf-name argument specify the name of the VRFinstance to associate with subsequent IPv4 address family configurationmode commands.

Step 11 neighbor {ip-address| peer-group-name}weight number

Example:

Router(config-router-af)# neighbor 192.168.3.2 weight 100

Assigns a weight to a BGP peer connection.

• In this example, the weight attribute for routes received from theBGP peer 192.168.3.2 is set to 100.

Step 12 end

Example:


Exits address family configuration mode and enters privileged EXECmode.

Step 13 clear ip bgp {*| ip-address| peer-group-name} [soft [in | out]]

Example:

Router# clear ip bgp *

(Optional) Clears BGP outbound route filters and initiates an outboundsoft reset. A single neighbor or all neighbors can be specified.


Example:

Router# show ip bgp


• Enter this command at Router B to see the weight attribute for eachroute to a BGP peer. The route with the highest weight attribute willbe the preferred route to network 172.17.1.0.

Note Only the syntax applicable to this task is used in this example. Formore details, see the Cisco IOS IP Routing: BGP CommandReference.

ExamplesThe following example shows the BGP routing table at Router B with the weight attributes assigned toroutes. The route through 192.168.3.2 (Router E in the figure above) has the highest weight attribute andwill be the preferred route to network 172.17.1.0.

BGP table version is 8, local router ID is 172.17.1.99

Configuring BGP Peering with ISPsConfiguring Multihoming with Two ISPs

33

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 10.1.1.0/24 192.168.1.2 0 100 40000 i*> 10.2.2.0/24 192.168.3.2 0 150 50000 i*> 172.17.1.0/24 0.0.0.0 0 32768 i

Multihoming with a Single ISPPerform this task to configure your network to access one of two connections to a single ISP, where one ofthe connections is the preferred route and the second connection is a backup route. In the figure aboveRouter E in autonomous system 50000 has two BGP peers in a single autonomous system, autonomoussystem 45000. Using this task, autonomous system 50000 does not learn any routes from autonomoussystem 45000 and is sending its own routes using BGP. This task is configured at Router E in the figureabove and covers three features about multihoming to a single ISP:

• Outbound traffic--Router E will forward default routes and traffic to autonomous system 45000 withRouter B as the primary link and Router D as the backup link. Static routes are configured to bothRouter B and Router D with a lower distance configured for the link to Router B.

• Inbound traffic--Inbound traffic from autonomous system 45000 is configured to be sent from RouterB unless the link fails when the backup route is to send traffic from Router D. To achieve this,outbound filters are set using the MED metric.

• Prevention of transit traffic--A route map is configured at Router E in autonomous system 50000 toblock all incoming BGP routing updates to prevent autonomous system 50000 from receiving transittraffic from the ISP in autonomous system 45000.

Configuring BGP Peering with ISPs Multihoming with a Single ISP

34

SUMMARY STEPS

1. enable







8. Repeat Step 7 to apply another route map to the neighbor specified in Step 7.

9. exit




13. Repeat Step 10 to apply another route map to the neighbor specified in Step 10.

14. exit

15. exit

16. ip route prefix mask {ip-address | interface-type interface-number[ip-address]} [distance] [name][permanent| track number][tag tag]

17. Repeat Step 14 to establish another static route.



20. exit



23. exit


25. end

26. show ip route [ip-address] [mask] [longer-prefixes]


DETAILED STEPS


Step 1 enable

Example:

Router> enable



Configuring BGP Peering with ISPsMultihoming with a Single ISP

35



Example:




Example:




Example:



• In this example, the BGP peer at Router D is added to the BGProuting table.


Example:







Example:





36



Example:

Router(config-router-af)# neighbor 192.168.2.1 route-map BLOCK in

Example:

Router(config-router-af)# neighbor 192.168.2.1 route-map SETMETRIC1 out


• In the first example, the route map named BLOCK is applied toinbound routes at Router E.

• In the second example, the route map named SETMETRIC1 isapplied to outbound routes to Router D.

Note Two examples are shown here because the task examplerequires both these statements to be configured.

Step 8 Repeat Step 7 to apply another route map to theneighbor specified in Step 7.

--

Step 9 exit

Example:




Example:



• In this example, the BGP peer at Router D is added to the BGProuting table.


Example:





The vrf keyword and vrf-name argument specify the name of the VRFinstance to associate with subsequent IPv4 address familyconfiguration mode commands.


37



Example:

Router(config-router-af)# neighbor 192.168.3.1 route-map BLOCK in

Example:

Router(config-router-af)# neighbor 192.168.3.1 route-map SETMETRIC2 out


• In the first example, the route map named BLOCK is applied toinbound routes at Router E.

• In the second example, the route map named SETMETRIC2 isapplied to outbound routes to Router D.


Step 13 Repeat Step 10 to apply another route map tothe neighbor specified in Step 10.

--

Step 14 exit

Example:



Step 15 exit

Example:




38


Step 16 ip route prefix mask {ip-address | interface-typeinterface-number[ip-address]} [distance][name][permanent| track number][tag tag]

Example:

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.2.1 50

Example:


Example:

and

Example:


Establishes a static route.

• In the first example, a static route to BGP peer 192.168.2.1 isestablished and given an administrative distance of 50.

• In the second example, a static route to BGP peer 192.168.3.1 isestablished and given an administrative distance of 40. The loweradministrative distance makes this route via Router B thepreferred route.


Step 17 Repeat Step 14 to establish another static route. --


Example:

Router(config)# route-map SETMETRIC1 permit 10


• In this example, a route map named SETMETRIC1 is created.


Example:



Step 20 exit

Example:

Router(config-route-map)# exit

Exits route map configuration mode and enters global configurationmode.


39



Example:

Router(config)# route-map SETMETRIC2 permit 10


• In this example, a route map named SETMETRIC2 is created.


Example:



Step 23 exit

Example:


Exits route map configuration mode and enters global configurationmode.


Example:

Router(config)# route-map BLOCK deny 10


• In this example, a route map named BLOCK is created to blockall incoming routes from autonomous system 45000.

Step 25 end

Example:



Step 26 show ip route [ip-address] [mask] [longer-prefixes]

Example:

Router# show ip route

(Optional) Displays route information from the routing tables.

• Use this command at Router E in the figure above after Router Band Router D have received update information containing theMED metric from Router E.

• Only the syntax applicable to this task is used in this example. Formore details, see the Cisco IOS IP Routing: BGP CommandReference.


40



Example:


(Optional) Displays the entries in the BGP routing table.

• Use this command at Router E in the figure above after Router Band Router D have received update information containing theMED metric from Router E.

• Only the syntax applicable to this task is used in this example. Formore details, see the Cisco IOS IP Routing: BGP CommandReference.

Examples

The following example shows output from the show ip route command entered at Router E after this taskhas been configured and Router B and Router D have received update information containing the MEDmetric. Note that the gateway of last resort is set as 192.168.3.1, which is the route to Router B.

Router# show ip routeCodes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static routeGateway of last resort is 192.168.3.1 to network 0.0.0.0 10.0.0.0/24 is subnetted, 1 subnetsC 10.2.2.0 is directly connected, Ethernet0/0C 192.168.2.0/24 is directly connected, Serial3/0C 192.168.3.0/24 is directly connected, Serial2/0S* 0.0.0.0/0 [40/0] via 192.168.3.1

The following example shows output from the show ip bgp command entered at Router E after this taskhas been configured and Router B and Router D have received routing updates. The route map BLOCK hasdenied all routes coming in from autonomous system 45000 so the only network shown is the localnetwork.

Router# show ip bgpBGP table version is 2, local router ID is 10.2.2.99Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 10.2.2.0/24 0.0.0.0 0 32768 i

The following example shows output from the show ip bgp command entered at Router B after this taskhas been configured at Router E and Router B has received routing updates. Note the metric of 50 fornetwork 10.2.2.0.

Router# show ip bgpBGP table version is 7, local router ID is 172.17.1.99Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 10.1.1.0/24 192.168.1.2 0 0 40000 i*> 10.2.2.0/24 192.168.3.2 50 0 50000 i*> 172.16.1.0/24 0.0.0.0 0 32768 i*> 172.17.1.0/24 0.0.0.0 0 32768 i


41

The following example shows output from the show ip bgp command entered at Router D after this taskhas been configured at Router E and Router D has received routing updates. Note the metric of 100 fornetwork 10.2.2.0.

Router# show ip bgpBGP table version is 3, local router ID is 192.168.2.1Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 10.2.2.0/24 192.168.2.2 100 0 50000 i*> 172.16.1.0/24 0.0.0.0 0 32768 i

Configuring Multihoming to Receive the Full Internet Routing TablePerform this task to configure your network to build neighbor relationships with other routers in otherautonomous systems while filtering outbound routes. In this task the full Internet routing table will bereceived from the service providers in the neighboring autonomous systems but only locally originatedroutes will be advertised to the service providers. This task is configured at Router B in the figure aboveand uses an access list to permit only locally originated routes and a route map to ensure that only thelocally originated routes are advertised outbound to other autonomous systems.

Note Be aware that receiving the full Internet routing table from two ISPs may use all the memory in smallerrouters.

SUMMARY STEPS

1. enable







8. exit




12. exit

13. exit

14. ip as-path access-list access-list-number {deny | permit} as-regular-expression


16. match as-path path-list-number

17. end


Configuring BGP Peering with ISPs Configuring Multihoming to Receive the Full Internet Routing Table

42

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:




Example:




Example:




Example:





• The vrf keyword and vrf-name argument specify the name of theVRF instance to associate with subsequent IPv4 address familyconfiguration mode commands.


Example:




Configuring BGP Peering with ISPsConfiguring Multihoming to Receive the Full Internet Routing Table

43



Example:

Router(config-router-af)# neighbor 192.168.1.2 route-map localonly out


• In this example, the route map named localonly is applied tooutbound routes to Router A.

Step 8 exit

Example:




Example:




Example:





The vrf keyword and vrf-name argument specify the name of theVRF instance to associate with subsequent IPv4 address familyconfiguration mode commands.


Example:

Router(config-router-af)# neighbor 192.168.3.2 route-map localonly out


• In this example, the route map named localonly is applied tooutbound routes to Router E.

Configuring BGP Peering with ISPs Configuring Multihoming to Receive the Full Internet Routing Table

44


Step 12 exit

Example:



Step 13 exit

Example:



Step 14 ip as-path access-list access-list-number {deny |permit} as-regular-expression

Example:

Router(config)# ip as-path access-list 10 permit ^$

Defines a BGP-related access list.

• In this example, the access list number 10 is defined to permitonly locally originated BGP routes.


Example:

Router(config)# route-map localonly permit 10


• In this example, a route map named localonly is created.

Step 16 match as-path path-list-number

Example:

Router(config-route-map)# match as-path 10

Matches a BGP autonomous system path access list.

• In this example, the BGP autonomous system path access listcreated in Step 12 is used for the match clause.

Step 17 end

Example:




Example:

Router# show ip bgp


Note Only the syntax applicable to this task is used in this example.For more details, see the Cisco IOS IP Routing: BGPCommand Reference.

Configuring BGP Peering with ISPsConfiguring Multihoming to Receive the Full Internet Routing Table

45

ExamplesThe following example shows the BGP routing table for Router B in the figure above after this task hasbeen configured. Note that the routing table contains the information about the networks in the autonomoussystems 40000 and 50000.

BGP table version is 5, local router ID is 172.17.1.99Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 10.1.1.0/24 192.168.1.2 0 0 40000 i*> 10.2.2.0/24 192.168.3.2 0 0 50000 i*> 172.17.1.0/24 0.0.0.0 0 32768 i

Configuring BGP PoliciesThe tasks in this section help you configure BGP policies that filter the traffic in your BGP network. Thefollowing optional tasks demonstrate some of the various methods by which traffic can be filtered in yourBGP network:

• Filtering BGP Prefixes with Prefix Lists, page 46• Filtering BGP Prefixes with AS-path Filters, page 50• Filtering BGP Prefixes with AS-path Filters Using 4-Byte Autonomous System Numbers, page 53• Filtering Traffic Using Community Lists, page 57• Filtering Traffic Using Extended Community Lists, page 62• Filtering Traffic Using a BGP Route Map Policy List, page 66• Filtering Traffic Using Continue Clauses in a BGP Route Map, page 70

Filtering BGP Prefixes with Prefix ListsPerform this task to use prefix lists to filter BGP route information. The task is configured at Router B inthe figure below where both Router A and Router E are set up as BGP peers. A prefix list is configured topermit only routes from the network 10.2.2.0/24 to be outbound. In effect, this will restrict the informationthat is received from Router E to be forwarded to Router A. Optional steps are included to display theprefix list information and to reset the hit count.

Figure 7 BGP Topology for Configuring BGP Policies Tasks

Configuring BGP Policies Filtering BGP Prefixes with Prefix Lists

46

Note The neighbor prefix-list and the neighbor distribute-list commands are mutually exclusive for a BGPpeer.

>

SUMMARY STEPS

1. enable



4. neighbor ip-address remote-as autonomous-system-number

5. Repeat Step 5 for all BGP peers.



8. aggregate-address address mask [as-set]

9. neighbor ip-address prefix-list list-name {in | out}

10. exit

11. exit

12. ip prefix-list list-name [seq seq-number] {deny network / length| permit network / length}[ge ge-value] [le le-value] [eq eq-value]

13. end

14. show ip prefix-list [detail | summary] [prefix-list-name [seq seq-number | network / length [longer |first-match]]]

15. clear ip prefix-list {*| ip-address| peer-group-name} out

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:



Configuring BGP PoliciesFiltering BGP Prefixes with Prefix Lists

47



Example:



Step 4 neighbor ip-address remote-as autonomous-system-number

Example:


Adds the IP address of the neighbor in the specified autonomoussystem to the BGP neighbor table of the local router.

Step 5 Repeat Step 5 for all BGP peers. --


Example:





• The vrf keyword and vrf-name argument specify the name of theVRF instance to associate with subsequent IPv4 address familyconfiguration mode commands.


Example:


(Optional) Specifies a network as local to this autonomous system andadds it to the BGP routing table.


Step 8 aggregate-address address mask [as-set]

Example:

Router(config-router-af)# aggregate-address 172.0.0.0 255.0.0.0

Creates an aggregate entry in a BGP routing table.

• A specified route must exist in the BGP table.• Use the aggregate-address command with no keywords to

create an aggregate entry if any more-specific BGP routes areavailable that fall in the specified range.

Note Only partial syntax is used in this example. For more details,see the Cisco IOS IP Routing: BGP Command Reference.

Configuring BGP Policies Filtering BGP Prefixes with Prefix Lists

48


Step 9 neighbor ip-address prefix-list list-name {in |out}

Example:

Router(config-router-af)# neighbor 192.168.1.2 prefix-list super172 out

Distributes BGP neighbor information as specified in a prefix list.

• In this example, a prefix list called super172 is set for outgoingroutes to Router A.

Step 10 exit

Example:



Step 11 exit

Example:

Router(config-router) exit


Step 12 ip prefix-list list-name [seq seq-number] {denynetwork / length| permit network / length}[ge ge-value] [le le-value] [eq eq-value]

Example:

Router(config)# ip prefix-list super172 permit 172.0.0.0/8

Defines a BGP-related prefix list and enters access list configurationmode.

• In this example, the prefix list called super172 is defined topermit only route 172.0.0.0/8 to be forwarded.

• All other routes will be denied because there is an implicit denyat the end of all prefix lists.

Step 13 end

Example:

Router(config-access-list)# end

Exits access list configuration mode and enters privileged EXECmode.

Step 14 show ip prefix-list [detail | summary] [prefix-list-name [seq seq-number | network / length[longer | first-match]]]

Example:

Router# show ip prefix-list detail super172

Displays information about prefix lists.

• In this example, details of the prefix list named super172 will bedisplayed, including the hit count. Hit count is the number oftimes the entry has matched a route.

Configuring BGP PoliciesFiltering BGP Prefixes with Prefix Lists

49


Step 15 clear ip prefix-list {*| ip-address| peer-group-name} out

Example:

Router# clear ip prefix-list super172 out

Resets the hit count of the prefix list entries.

• In this example, the hit count for the prefix list called super172will be reset.

Examples

The following output from the show ip prefix-list command shows details of the prefix list namedsuper172, including the hit count. The clear ip prefix-listcommand is entered to reset the hit count and theshow ip prefix-list command is entered again to show the hit count reset to 0.

Router# show ip prefix-list detail super172ip prefix-list super172: count: 1, range entries: 0, sequences: 5 - 5, refcount: 4 seq 5 permit 172.0.0.0/8 (hit count: 1, refcount: 1)Router# clear ip prefix-list super172Router# show ip prefix-list detail super172ip prefix-list super172: count: 1, range entries: 0, sequences: 5 - 5, refcount: 4 seq 5 permit 172.0.0.0/8 (hit count: 0, refcount: 1)

Filtering BGP Prefixes with AS-path FiltersPerform this task to filter BGP prefixes using AS-path filters with an access list based on the value of theAS-path attribute to filter route information. An AS-path access list is configured at Router B in the figureabove. The first line of the access list denies all matches to the AS-path 50000 and the second line allowsall other paths. The router uses the neighbor filter-list command to specify the AS-path access list as anoutbound filter. After the filtering is enabled, traffic can be received from both Router A and Router C butupdates originating from autonomous system 50000 (Router C) are not forwarded by Router B to Router A.If any updates from Router C originated from another autonomous system, they would be forwardedbecause they would contain both autonomous system 50000 plus another autonomous system number, andthat would not match the AS-path access list.


Configuring BGP Policies Filtering BGP Prefixes with AS-path Filters

50

SUMMARY STEPS

1. enable







8. neighbor {ip-address | peer-group-name} filter-list access-list-number{in | out}

9. exit

10. exit


12. Repeat Step 11 for all entries required in the AS-path access list.

13. end

14. show ip bgp regexp as-regular-expression

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:




Example:




Example:


Adds the IP address or peer group name of the neighbor in thespecified autonomous system BGP neighbor table of the local router.

Configuring BGP PoliciesFiltering BGP Prefixes with AS-path Filters

51




Example:



• The unicast keyword specifies the IPv4 unicast address family.By default, the router is placed in address family configurationmode for the IPv4 unicast address family if the unicast keyword isnot specified with the address-family ipv4 command.




Example:




Note Only partial syntax is used in this example. For more details, seethe Cisco IOS IP Routing: BGP Command Reference.

Step 8 neighbor {ip-address | peer-group-name}filter-list access-list-number{in | out}

Example:

Router(config-router-af)# neighbor 192.168.1.2 filter-list 100 out


• In this example, an access list number 100 is set for outgoingroutes to Router A.

Step 9 exit

Example:


Exits address family configuration mode and returns to routerconfiguration mode.

Step 10 exit

Example:


Exits router configuration mode and returns to global configurationmode.

Configuring BGP Policies Filtering BGP Prefixes with AS-path Filters

52


Step 11 ip as-path access-list access-list-number {deny| permit} as-regular-expression

Example:

Router(config)# ip as-path access-list 100 deny ^50000$

Example:

Router(config)# ip as-path access-list 100 permit .*

Defines a BGP-related access list and enters access list configurationmode.

• In the first example, access list number 100 is defined to deny anyAS-path that starts and ends with 50000.

• In the second example, all routes that do not match the criteria inthe first example of the AS-path access list will be permitted. Theperiod and asterisk symbols imply that all characters in the AS-path will match, so Router B will forward those updates to RouterA.

Note Two examples are shown here because the task example requiresboth these statements to be configured.

Step 12 Repeat Step 11 for all entries required in theAS-path access list.

--

Step 13 end

Example:


Exits access list configuration mode and returns to privileged EXECmode.

Step 14 show ip bgp regexp as-regular-expression

Example:

Router# show ip bgp regexp ^50000$

Displays routes that match the regular expression.

• To verify the regular expression, you can use this command.• In this example, all paths that match the expression "starts and

ends with 50000" will be displayed.

Examples

The following output from the show ip bgp regexp command shows the autonomous system paths thatmatch the regular expression--start and end with AS-path 50000:

Router# show ip bgp regexp ^50000$BGP table version is 9, local router ID is 172.17.1.99Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 10.2.2.0/24 192.168.3.2 0 150 50000 i

Filtering BGP Prefixes with AS-path Filters Using 4-Byte Autonomous System NumbersIn Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)SXI1, and later releases, BGPsupport for 4-octet (4-byte) autonomous system numbers was introduced. The 4-byte autonomous systemnumbers in this task are formatted in the default asplain (decimal value) format, for example, Router B is inautonomous system number 65538 in the figure below. For more details about the introduction of 4-byteautonomous system numbers, see BGP Autonomous System Number Formats, page 4.

Perform this task to filter BGP prefixes with AS-path filters using 4-byte autonomous system numbers withan access list based on the value of the AS-path attribute to filter route information. An AS-path access list

Configuring BGP PoliciesFiltering BGP Prefixes with AS-path Filters Using 4-Byte Autonomous System Numbers

53

is configured at Router B in the figure below. The first line of the access list denies all matches to the AS-path 65550 and the second line allows all other paths. The router uses the neighbor filter-list command tospecify the AS-path access list as an outbound filter. After the filtering is enabled, traffic can be receivedfrom both Router A and Router E but updates originating from autonomous system 65550 (Router E) arenot forwarded by Router B to Router A. If any updates from Router E originated from another autonomoussystem, they would be forwarded because they would contain both autonomous system 65550 plus anotherautonomous system number, and that would not match the AS-path access list.


Figure 8 BGP Topology for Filtering BGP Prefixes with AS-path Filters Using 4-Byte Autonomous SystemNumbers

Configuring BGP Policies Filtering BGP Prefixes with AS-path Filters Using 4-Byte Autonomous System Numbers

54

SUMMARY STEPS

1. enable







8. neighbor {ip-address | peer-group-name} filter-list access-list-number{in | out}

9. exit

10. exit


12. Repeat Step 11 for all entries required in the AS-path access list.

13. end

14. show ip bgp regexp as-regular-expression

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:




Example:




Example:

Router(config-router-af)# neighbor 192.168.1.2 remote-as 65536

Adds the IP address or peer group name of the neighbor in the specifiedautonomous system BGP neighbor table of the local router.

• In this example, the IP address for the neighbor at Router A isadded.

Configuring BGP PoliciesFiltering BGP Prefixes with AS-path Filters Using 4-Byte Autonomous System Numbers

55




Example:



• The unicast keyword specifies the IPv4 unicast address family. Bydefault, the router is placed in address family configuration modefor the IPv4 unicast address family if the unicast keyword is notspecified with the address-family ipv4 command.




Example:




Note Only partial syntax is used in this example. For more details, seethe Cisco IOS IP Routing: BGP Command Reference.

Step 8 neighbor {ip-address | peer-group-name}filter-list access-list-number{in | out}

Example:

Router(config-router-af)# neighbor 192.168.1.2 filter-list 99 out


• In this example, an access list number 99 is set for outgoing routesto Router A.

Step 9 exit

Example:


Exits address family configuration mode and returns to routerconfiguration mode.

Step 10 exit

Example:


Exits router configuration mode and returns to global configurationmode.

Configuring BGP Policies Filtering BGP Prefixes with AS-path Filters Using 4-Byte Autonomous System Numbers

56


Step 11 ip as-path access-list access-list-number{deny | permit} as-regular-expression

Example:

Router(config)# ip as-path access-list 99 deny ^65550$

Example:

and

Example:

Router(config)# ip as-path access-list 99 permit .*

Defines a BGP-related access list and enters access list configurationmode.

• In the first example, access list number 99 is defined to deny anyAS-path that starts and ends with 65550.

• In the second example, all routes that do not match the criteria inthe first example of the AS-path access list will be permitted. Theperiod and asterisk symbols imply that all characters in the AS-path will match, so Router B will forward those updates to RouterA.


Step 12 Repeat Step 11 for all entries required in theAS-path access list.

--

Step 13 end

Example:


Exits access list configuration mode and returns to privileged EXECmode.

Step 14 show ip bgp regexp as-regular-expression

Example:

Router# show ip bgp regexp ^65550$

Displays routes that match the regular expression.

• To verify the regular expression, you can use this command.• In this example, all paths that match the expression "starts and

ends with 65550" will be displayed.

Examples

The following output from the show ip bgp regexp command shows the autonomous system paths thatmatch the regular expression--start and end with AS-path 65550:

RouterB# show ip bgp regexp ^65550$BGP table version is 4, local router ID is 172.17.1.99Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 10.2.2.0/24 192.168.3.2 0 0 65550 i

Filtering Traffic Using Community ListsPerform this task to filter traffic by creating BGP community lists and then reference them within a routemap to control incoming routes. BGP communities provide a method of filtering inbound or outboundroutes for large, complex networks. Instead of compiling long access or prefix lists of individual peers,

Configuring BGP PoliciesFiltering Traffic Using Community Lists

57

BGP allows grouping of peers with identical routing policies even though they reside in differentautonomous systems or networks.

In this task, Router B in the figure above is configured with several route maps and community lists tocontrol incoming routes.

SUMMARY STEPS

1. enable





6. neighbor {ip-address | peer-group-name} route-map route-map-name{in | out}

7. exit

8. exit

9. route-map map-name [permit | deny] [sequence-number]

10. match community {standard-list-number | expanded-list-number | community-list-name [exact]}

11. set weight weight

12. exit


14. match community {standard-list-number | expanded-list-number | community-list-name [exact]}

15. set community community-number

16. exit

17. ip community-list {standard-list-number| standard list-name {deny | permit} [community-number][AA:NN] [internet] [local-AS] [no-advertise] [no-export]} | {expanded-list-number | expanded list-name {deny | permit} regular-expression}

18. Repeat Step 15 to create all the required community lists.

19. exit

20. show ip community-list [standard-list-number | expanded-list-number | community-list-name][exact-match]

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:



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Example:



Step 4 neighbor {ip-address | peer-group-name} remote-as autonomous-system-number

Example:


Adds the IP address or peer group name of the neighbor in thespecified autonomous system BGP neighbor table of the localrouter.

Step 5 address-family ipv4 [unicast | multicast| vrf vrf-name]

Example:



• The unicast keyword specifies the IPv4 unicast addressfamily. By default, the router is placed in address familyconfiguration mode for the IPv4 unicast address family if theunicast keyword is not specified with the address-familyipv4 command.


• The vrf keyword and vrf-name argument specify the name ofthe VRF instance to associate with subsequent IPv4 addressfamily configuration mode commands.

Step 6 neighbor {ip-address | peer-group-name} route-map route-map-name{in | out}

Example:

Router(config-router-af)# neighbor 192.168.3.2 route-map 2000 in

Applies a route map to inbound or outbound routes.

• In this example, the route map called 2000 is applied toinbound routes from the BGP peer at 192.168.3.2.

Step 7 exit

Example:



Step 8 exit

Example:




59


Step 9 route-map map-name [permit | deny] [sequence-number]

Example:

Router(config)# route-map 2000 permit 10

Creates a route map and enters route map configuration mode.

• In this example, the route map called 2000 is defined.

Step 10 match community {standard-list-number |expanded-list-number | community-list-name[exact]}

Example:

Router(config-route-map)# match community 1

Matches a BGP community list.

• In this example, the community attribute is matched tocommunity list 1.

Step 11 set weight weight

Example:

Router(config-route-map)# set weight 30

Specifies the BGP weight for the routing table.

• In this example, any route that matches community list 1 willhave the BGP weight set to 30.

Step 12 exit

Example:


Exits route map configuration mode and enters globalconfiguration mode.


Example:

Router(config)# route-map 3000 permit 10

Creates a route map and enters route map configuration mode.

• In this example, the route map called 3000 is defined.

Step 14 match community {standard-list-number |expanded-list-number | community-list-name[exact]}

Example:

Router(config-route-map)# match community 2

Matches a BGP community list.

• In this example, the community attribute is matched tocommunity list 2.

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Step 15 set community community-number

Example:

Router(config-route-map)# set community 99

Sets the BGP communities attribute.

• In this example, any route that matches community list 2 willhave the BGP community attribute set to 99.

Step 16 exit

Example:


Exits route map configuration mode and enters globalconfiguration mode.

Step 17 ip community-list {standard-list-number| standardlist-name {deny | permit} [community-number][AA:NN] [internet] [local-AS] [no-advertise] [no-export]} | {expanded-list-number | expanded list-name {deny | permit} regular-expression}

Example:

Router(config)# ip community-list 1 permit 100

Example:

and

Example:

Router(config)# ip community-list 2 permit internet

Creates a community list for BGP and controls access to it.

• In the first example, community list 1 permits routes with acommunity attribute of 100. Router C routes all havecommunity attribute of 100 so their weight will be set to 30.

• In the second example, community list 2 effectively permitsall routes by using the internetkeyword. Any routes that didnot match community list 1 are checked against communitylist 2. All routes are permitted but no changes are made to theroute attributes.


Step 18 Repeat Step 15 to create all the required communitylists.

--

Step 19 exit

Example:

Router(config)# exit

Exits global configuration mode and enters privileged EXECmode.


61


Step 20 show ip community-list [standard-list-number |expanded-list-number | community-list-name][exact-match]

Example:

Router# show ip community-list 1

Displays configured BGP community list entries.

Examples

The following sample output verifies that community list 1 has been created, with the output showing thatcommunity list 1 permits routes with a community attribute of 100:

Router# show ip community-list 1Community standard list 1 permit 100

The following sample output verifies that community list 2 has been created, with the output showing thatcommunity list 2 effectively permits all routes by using the internetkeyword:

Router# show ip community-list 2Community standard list 2 permit internet

Filtering Traffic Using Extended Community ListsPerform this task to filter traffic by creating an extended BGP community list to control outbound routes.BGP communities provide a method of filtering inbound or outbound routes for large, complex networks.Instead of compiling long access or prefix lists of individual peers, BGP allows grouping of peers withidentical routing policies even though they reside in different autonomous systems or networks.

In this task, Router B in the figure above is configured with an extended named community list to specifythat the BGP peer at 192.168.1.2 is not sent advertisements about any path through or from autonomoussystem 50000. The IP extended community-list configuration mode is used and the ability to resequenceentries is shown.

Note A sequence number is applied to all extended community list entries by default regardless of theconfiguration mode. Explicit sequencing and resequencing of extended community list entries can beconfigured only in IP extended community-list configuration mode and not in global configuration mode.

>

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62

SUMMARY STEPS

1. enable


3. ip extcommunity-list {expanded-list-number| expanded list-name| standard-list-number | standardlist-name}

4. [sequence-number] {deny[regular-expression] | exit | permit[regular-expression]}

5. Repeat Step 4 for all the required permit or deny entries in the extended community list.

6. resequence [starting-sequence][sequence-increment]

7. exit



10. Repeat Step 10 for all the required BGP peers.



13. end

14. show ip extcommunity-list [list-name]

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:



Step 3 ip extcommunity-list {expanded-list-number|expanded list-name| standard-list-number |standard list-name}

Example:

Router(config)# ip extcommunity-list expanded DENY50000

Enters IP extended community-list configuration mode to create orconfigure an extended community list.

• In this example, the expanded community list DENY50000 iscreated.

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63


Step 4 [sequence-number] {deny[regular-expression]| exit | permit[regular-expression]}

Example:

Router(config-extcomm-list)# 10 deny _50000_

Example:

and

Example:

Router(config-extcomm-list)# 20 deny ^50000 .*

Configures an expanded community list entry.

• In the first example, an expanded community list entry with thesequence number 10 is configured to deny advertisements aboutpaths from autonomous system 50000.

• In the second example, an expanded community list entry with thesequence number 20 is configured to deny advertisements aboutpaths through autonomous system 50000.



Step 5 Repeat Step 4 for all the required permit ordeny entries in the extended community list.

--

Step 6 resequence [starting-sequence][sequence-increment]

Example:

Router(config-extcomm-list)# resequence 50 100

Resequences expanded community list entries.

• In this example, the sequence number of the first expandedcommunity list entry is set to 50 and subsequent entries are set toincrement by 100. The second expanded community list entry istherefore set to 150.


Step 7 exit

Example:

Router(config-extcomm-list)# exit

Exits expanded community-list configuration mode and enters globalconfiguration mode.


Example:



Configuring BGP Policies Filtering Traffic Using Extended Community Lists

64



Example:


Adds the IP address or peer group name of the neighbor in the specifiedautonomous system BGP neighbor table of the local router.

Step 10 Repeat Step 10 for all the required BGP peers. --


Example:



• The unicast keyword specifies the IPv4 unicast address family. Bydefault, the router is placed in address family configuration modefor the IPv4 unicast address family if the unicast keyword is notspecified with the address-family ipv4 command.


Note The vrf keyword and vrf-name argument specify the name of theVRF instance to associate with subsequent IPv4 address familyconfiguration mode commands.


Example:





Step 13 end

Example:


Exits address family configuration mode and enters privileged EXECmode.

Step 14 show ip extcommunity-list [list-name]

Example:

Router# show ip extcommunity-list DENY50000

Displays configured BGP expanded community list entries.

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Examples

The following sample output verifies that the BGP expanded community list DENY50000 has beencreated, with the output showing that the entries to deny advertisements about autonomous system 50000have been resequenced from 10 and 20 to 50 and 150:

Router# show ip extcommunity-list DENY50000Expanded extended community-list DENY50000 50 deny _50000_ 150 deny ^50000 .*

Filtering Traffic Using a BGP Route Map Policy ListPerform this task to create a BGP policy list and then reference it within a route map.

A policy list is like a route map that contains only match clauses. With policy lists there are no changes tomatch clause semantics and route map functions. The match clauses are configured in policy lists withpermit and deny statements and the route map evaluates and processes each match clause to permit or denyroutes based on the configuration. AND and OR semantics in the route map function the same way forpolicy lists as they do for match clauses.

Policy lists simplify the configuration of BGP routing policy in medium-size and large networks. Thenetwork operator can reference preconfigured policy lists with groups of match clauses in route maps andeasily apply general changes to BGP routing policy. The network operator no longer needs to manuallyreconfigure each recurring group of match clauses that occur in multiple route map entries.

Perform this task to create a BGP policy list to filter traffic that matches the autonomous system path andMED of a router and then create a route map to reference the policy list.

BGP routing must be configured in your network and BGP neighbors must be established.

Note• BGP route map policy lists do not support the configuration of IP version 6 (IPv6) match clauses in

policy lists.• Policy lists are not supported in versions of Cisco IOS software prior to Cisco IOS Releases 12.0(22)S

and 12.2(15)T. Reloading a router that is running an older version of Cisco IOS software may causesome routing policy configurations to be lost.

• Policy lists support only match clauses and do not support set clauses. However, policy lists cancoexist, within the same route map entry, with match and set clauses that are configured separatelyfrom the policy lists.

• Policy lists are supported only by BGP. They are not supported by other IP routing protocols. Thislimitation does not interfere with normal operations of a route map, including redistribution, becausepolicy list functions operate transparently within BGP and are not visible to other IP routing protocols.

• Policy lists support only match clauses and do not support set clauses. However, policy lists cancoexist, within the same route map entry, with match and set clauses that are configured separatelyfrom the policy lists. The first route map example configures AND semantics, and the second routemap configuration example configures semantics. Both examples in this section show sample routemap configurations that reference policy lists and separate match and set clauses in the sameconfiguration.

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66

SUMMARY STEPS

1. enable


3. ip policy-list policy-list-name {permit | deny}

4. match as-path as-number

5. match metric metric

6. exit


8. match ip address {access-list-number | access-list-name} [... access-list-number | ... access-list-name]

9. match policy-list policy-list-name

10. set community community-number [additive] [well-known-community] | none}

11. set local-preference preference-value

12. end

13. show ip policy-list [policy-list-name]

14. show route-map [route-map-name]

DETAILED STEPS


Step 1 enable

Example:

Router> enable




Example:



Step 3 ip policy-list policy-list-name {permit | deny}

Example:

Router(config)# ip policy-list POLICY-LIST-NAME-1 permit

Enters policy list configuration mode and creates a BGPpolicy list that will permit routes that are allowed by thematch clauses that follow.

Step 4 match as-path as-number

Example:

Router(config-policy-list)# match as-path 500

Creates a match clause to permit routes from thespecified autonomous system path.

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67


Step 5 match metric metric

Example:

Router(config-policy-list)# match metric 10

Creates a match clause to permit routes with thespecified metric.

Step 6 exit

Example:

Router(config-policy-list)# exit

Exits policy list configuration mode and enters globalconfiguration mode.


Example:

Router(config)# route-map MAP-NAME-1 permit 10

Creates a route map and enters route map configurationmode.

Step 8 match ip address {access-list-number | access-list-name} [...access-list-number | ... access-list-name]

Example:

Router(config-route-map)# match ip address 1

Creates a match clause to permit routes that match thespecified access-list-numberor access-list-nameargument.

Step 9 match policy-list policy-list-name

Example:

Router(config-route-map)# match policy-list POLICY-LIST-NAME-1

Creates a clause that will match the specified policy list.

• All match clauses within the policy list will beevaluated and processed. Multiple policy lists canreferenced with this command.

• This command also supports AND or OR semanticslike a standard match clause.

Step 10 set community community-number [additive] [well-known-community] | none}

Example:

Router(config-route-map)# set community 10:1

Creates a clause to set or remove the specifiedcommunity.

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Step 11 set local-preference preference-value

Example:

Router(config-route-map)# set local-preference 140

Creates a clause to set the specified local preferencevalue.

Step 12 end

Example:


Exits route map configuration mode and entersprivileged EXEC mode.

Step 13 show ip policy-list [policy-list-name]

Example:

Router# show ip policy-list POLICY-LIST-NAME-1

Display information about configured policy lists andpolicy list entries.

Step 14 show route-map [route-map-name]

Example:

Router# show route-map

Displays locally configured route maps and route mapentries.

Examples

The following sample output verifies that a policy list has been created, with the output displaying thepolicy list name and configured match clauses:

Router# show ip policy-list POLICY-LIST-NAME-1policy-list POLICY-LIST-NAME-1 permit Match clauses: metric 20 as-path (as-path filter): 1

Note A policy list name can be specified when the show ip policy-list command is entered. This option can beuseful for filtering the output of this command and verifying a single policy list.

The following sample output from the show route-map command verifies that a route map has beencreated and a policy list is referenced. The output of this command displays the route map name and policylists that are referenced by the configured route maps.

Router# show route-maproute-map ROUTE-MAP-NAME-1, deny, sequence 10 Match clauses: Set clauses: Policy routing matches: 0 packets, 0 bytesroute-map ROUTE-MAP-NAME-1, permit, sequence 10

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69

Match clauses: IP Policy lists: POLICY-LIST-NAME-1 Set clauses: Policy routing matches: 0 packets, 0 bytes

Filtering Traffic Using Continue Clauses in a BGP Route MapPerform this task to filter traffic using continue clauses in a BGP route map.

SUMMARY STEPS

1. enable





6. neighbor {ip-address| peer-group-name} route-map map-name{in | out}

7. exit

8. exit

9. route-map map-name {permit | deny} [sequence-number]

10. match ip address {access-list-number | access-list-name} [... access-list-number | ... access-list-name]

11. set community community-number [additive] [well-known-community] | none}

12. continue [sequence-number]

13. end

14. show route-map [map-name]

DETAILED STEPS


Step 1 enable

Example:

Router> enable

Enables higher privilege levels, such as privileged EXEC mode.



Example:




Example:



Configuring BGP Policies Filtering Traffic Using Continue Clauses in a BGP Route Map

70



Example:



Step 5 address-family ipv4 [unicast | multicast|vrf vrf-name]

Example:


Specifies the IPv4 address family and enters address family configurationmode.

• The unicast keyword specifies the IPv4 unicast address family. Bydefault, the router is placed in address family configuration mode forthe IPv4 unicast address family if the unicast keyword is notspecified with the address-family ipv4 command.


The vrf keyword and vrf-name argument specify the name of the VRFinstance to associate with subsequent IPv4 address family configurationmode commands.

Step 6 neighbor {ip-address| peer-group-name}route-map map-name{in | out}

Example:

Router(config-router-af)# neighbor 10.0.0.1 route-map ROUTE-MAP-NAME in

Applies the inbound route map to routes received from the specifiedneighbor, or applies an outbound route map to routes advertised to thespecified neighbor.

Step 7 exit

Example:


Exits address family configuration mode and enters router configurationmode.

Step 8 exit

Example:



Step 9 route-map map-name {permit | deny}[sequence-number]

Example:

Router(config)# route-map ROUTE-MAP-NAME permit 10

Enters route-map configuration mode to create or configure a route map.

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71


Step 10 match ip address {access-list-number |access-list-name} [... access-list-number | ...access-list-name]

Example:

Router(config-route-map)# match ip address 1

Configures a match command that specifies the conditions under whichpolicy routing and route filtering occur.

• Multiple match commands can be configured. If a match commandis configured, a match must occur in order for the continue statementto be executed. If a match command is not configured, set andcontinue clauses will be executed.

Note The match and set commands used in this task are examples thatare used to help describe the operation of the continue command.For a list of specific match and set commands, see the continuecommand in the Cisco IOS IP Routing: BGP Command Reference.

Step 11 set community community-number[additive] [well-known-community] | none}

Example:

Router(config-route-map)# set community 10:1

Configures a set command that specifies the routing action to perform ifthe criteria enforced by the match commands are met.

• Multiple set commands can be configured.• In this example, a clause is created to set the specified community.

Step 12 continue [sequence-number]

Example:

Router(config-route-map)# continue

Configures a route map to continue to evaluate and execute matchstatements after a successful match occurs.

• If a sequence number is configured, the continue clause will go to theroute map with the specified sequence number.

• If no sequence number is specified, the continue clause will go to theroute map with the next sequence number. This behavior is called an"implied continue."

Note Continue clauses in outbound route maps are supported only inCisco IOS Release 12.0(31)S, 12.2(33)SB, 12.2(33)SRB,12.2(33)SXI, 12.4(4)T, and later releases.

Step 13 end

Example:


Exits route-map configuration mode and enters privileged EXEC mode.

Step 14 show route-map [map-name]

Example:

Router# show route-map

(Optional) Displays locally configured route maps. The name of the routemap can be specified in the syntax of this command to filter the output.

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72

Examples

The following sample output shows how to verify the configuration of continue clauses using the showroute-map command. The output displays configured route maps including the match, set, and continueclauses.

Router# show route-maproute-map MARKETING, permit, sequence 10 Match clauses: ip address (access-lists): 1 metric 10 Continue: sequence 40 Set clauses: as-path prepend 10 Policy routing matches: 0 packets, 0 bytesroute-map MARKETING, permit, sequence 20 Match clauses: ip address (access-lists): 2 metric 20 Set clauses: as-path prepend 10 10 Policy routing matches: 0 packets, 0 bytesroute-map MARKETING, permit, sequence 30 Match clauses: Continue: to next entry 40 Set clauses: as-path prepend 10 10 10 Policy routing matches: 0 packets, 0 bytesroute-map MARKETING, permit, sequence 40 Match clauses: community (community-list filter): 10:1 Set clauses: local-preference 104 Policy routing matches: 0 packets, 0 bytesroute-map MKTG-POLICY-MAP, permit, sequence 10 Match clauses: Set clauses: community 655370 Policy routing matches: 0 packets, 0 bytes

Configuration Examples for Connecting to a Service ProviderUsing External BGP

• Influencing Inbound Path Selection Examples, page 74

• Influencing Inbound Path Selection by Modifying the AS-path Attribute Using 4-Byte AS NumbersExample, page 74

• Influencing Outbound Path Selection Examples, page 76

• Filtering BGP Prefixes with Prefix Lists Examples, page 77

• Filtering Traffic Using Community Lists Examples, page 78

• Filtering Traffic Using AS-path Filters Example, page 79

• Filtering Traffic with AS-path Filters Using 4-Byte Autonomous System Numbers Examples, page79

• Filtering Traffic Using Extended Community Lists with 4-Byte Autonomous System NumbersExample, page 80

• Filtering Traffic Using a BGP Route Map Example, page 83

• Filtering Traffic Using Continue Clauses in a BGP Route Map Examples, page 83

Configuring BGP PoliciesConfiguration Examples for Connecting to a Service Provider Using External BGP

73

Influencing Inbound Path Selection ExamplesThe following example shows how you can use route maps to modify incoming data from a neighbor. Anyroute received from 10.222.1.1 that matches the filter parameters set in autonomous system access list 200will have its weight set to 200 and its local preference set to 250, and it will be accepted.

router bgp 100! neighbor 10.222.1.1 route-map FIX-WEIGHT in neighbor 10.222.1.1 remote-as 1!ip as-path access-list 200 permit ^690$ip as-path access-list 200 permit ^1800!route-map FIX-WEIGHT permit 10 match as-path 200 set local-preference 250 set weight 200

In the following example, the route map named finance marks all paths originating from autonomoussystem 690 with an MED metric attribute of 127. The second permit clause is required so that routes notmatching autonomous system path list 1 will still be sent to neighbor 10.1.1.1.

router bgp 65000 neighbor 10.1.1.1 route-map finance out!ip as-path access-list 1 permit ^690_ip as-path access-list 2 permit .*!route-map finance permit 10 match as-path 1 set metric 127!route-map finance permit 20 match as-path 2

Inbound route maps could perform prefix-based matching and set various parameters of the update.Inbound prefix matching is available in addition to autonomous system path and community list matching.The following example shows how the set local-preference route map configuration command sets thelocal preference of the inbound prefix 172.20.0.0/16 to 120:

!router bgp 65100 network 10.108.0.0 neighbor 10.108.1.1 remote-as 65200 neighbor 10.108.1.1 route-map set-local-pref in !route-map set-local-pref permit 10 match ip address 2 set local preference 120!route-map set-local-pref permit 20!access-list 2 permit 172.20.0.0 0.0.255.255access-list 2 deny any

Influencing Inbound Path Selection by Modifying the AS-path AttributeUsing 4-Byte AS Numbers Example

This example shows how to configure BGP to influence the inbound path selection for traffic destined forthe 172.17.1.0 network by modifying the AS-path attribute. In Cisco IOS Release 12.0(32)SY8,12.0(33)S3, 12.2(33)SXI1, and later releases, BGP support for 4-octet (4-byte) autonomous system

Influencing Inbound Path Selection Examples Configuration Examples for Connecting to a Service Provider Using External BGP

74

numbers was introduced. The 4-byte autonomous system numbers in this example are formatted in thedefault asplain (decimal value) format; for example, Router B is in autonomous system number 65538 inthe figure below. For more details about the introduction of 4-byte autonomous system numbers, see BGPAutonomous System Number Formats, page 4.One of the methods that BGP can use to influence the choice of paths in another autonomous system is tomodify the AS-path attribute. For example, in the figure below, Router A advertises its own network,172.17.1.0, to its BGP peers in autonomous system 65538 and autonomous system 65550. When therouting information is propagated to autonomous system 65545, the routers in autonomous system 65545have network reachability information about network 172.17.1.0 from two different routes. The first routeis from autonomous system 65538 with an AS-path consisting of 65538, 65536. The second route isthrough autonomous system 65547 with an AS-path of 65547, 65550, 65536. If all other BGP attributevalues are the same, Router C in autonomous system 65545 would choose the route through autonomoussystem 65538 for traffic destined for network 172.17.1.0 because it is the shortest route in terms ofautonomous systems traversed.Autonomous system 65536 now receives all traffic from autonomous system 65545 for the 172.17.1.0network through Router B in autonomous system 65538. If, however, the link between autonomous system65538 and autonomous system 65536 is a really slow and congested link, the set as-pathprependcommand can be used at Router A to influence inbound path selection for the 172.17.1.0 networkby making the route through autonomous system 65538 appear to be longer than the path throughautonomous system 65550. The configuration is done at Router A in the figure below by applying a routemap to the outbound BGP updates to Router B. Using the set as-path prependcommand, all the outboundBGP updates from Router A to Router B will have their AS-path attribute modified to add the localautonomous system number 65536 twice. After the configuration, autonomous system 65545 receivesupdates about the 172.17.1.0 network through autonomous system 65538. The new AS-path is 65538,65536, 65536, 65536, which is now longer than the AS-path from autonomous system 65547 (unchanged ata value of 65547, 65550, 65536). Networking devices in autonomous system 65545 will now prefer theroute through autonomous system 65547 to forward packets with a destination address in the 172.17.1.0network.Figure 9 Network Topology for Modifying the AS-path Attribute

Influencing Inbound Path Selection by Modifying the AS-path Attribute Using 4-Byte AS Numbers ExampleConfiguration Examples for Connecting to a Service Provider Using External BGP

75

The configuration for this example is performed at Router A in the figure above.

router bgp 65536 address-family ipv4 unicast network 172.17.1.0 mask 255.255.255.0 neighbor 192.168.1.2 remote-as 65538 neighbor 192.168.1.2 activate neighbor 192.168.1.2 route-map PREPEND out exit-address-family exitroute-map PREPEND permit 10set as-path prepend 65536 65536

Influencing Outbound Path Selection ExamplesThe following example creates an outbound route filter and configures Router-A (10.1.1.1) to advertise thefilter to Router-B (172.16.1.2). An IP prefix list named FILTER is created to specify the 192.168.1.0/24subnet for outbound route filtering. The ORF send capability is configured on Router-A so that Router-Acan advertise the outbound route filter to Router-B.

Router-A Configuration (Sender)

ip prefix-list FILTER seq 10 permit 192.168.1.0/24 !router bgp 65100 address-family ipv4 unicast neighbor 172.16.1.2 remote-as 65200 neighbor 172.16.1.2 ebgp-multihop neighbor 172.16.1.2 capability orf prefix-list send neighbor 172.16.1.2 prefix-list FILTER in end

Router-B Configuration (Receiver)

The following example configures Router-B to advertise the ORF receive capability to Router-A. Router-Bwill install the outbound route filter, defined in the FILTER prefix list, after ORF capabilities have beenexchanged. An inbound soft reset is initiated on Router-B at the end of this configuration to activate theoutbound route filter.

router bgp 65200 address-family ipv4 unicast neighbor 10.1.1.1 remote-as 65100 neighbor 10.1.1.1 ebgp-multihop 255 neighbor 10.1.1.1 capability orf prefix-list receive end clear ip bgp 10.1.1.1 in prefix-filter

The following example shows how the route map named set-as-path is applied to outbound updates to theneighbor 10.69.232.70. The route map will prepend the autonomous system path "65100 65100" to routesthat pass access list 1. The second part of the route map is to permit the advertisement of other routes.

router bgp 65100 network 172.16.0.0 network 172.17.0.0 neighbor 10.69.232.70 remote-as 65200 neighbor 10.69.232.70 route-map set-as-path out!route-map set-as-path 10 permit match address 1 set as-path prepend 65100 65100!route-map set-as-path 20 permit match address 2

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76

!access-list 1 permit 172.16.0.0 0.0.255.255access-list 1 permit 172.17.0.0 0.0.255.255!access-list 2 permit 0.0.0.0 255.255.255.255

Filtering BGP Prefixes with Prefix Lists ExamplesThis section contains the following examples:

• Filtering BGP Prefixes Using a Single Prefix List, page 77

• Filtering BGP Prefixes Using a Group of Prefixes, page 78

• Adding or Deleting Prefix List Entries, page 78

Filtering BGP Prefixes Using a Single Prefix ListThe following example shows how a prefix list denies the default route 0.0.0.0/0:

ip prefix-list abc deny 0.0.0.0/0

The following example shows how a prefix list permits a route that matches the prefix 10.0.0.0/8:

ip prefix-list abc permit 10.0.0.0/8

The following example shows how to configure the BGP process so that it accepts only prefixes with aprefix length of /8 to /24:

router bgp 40000 network 10.20.20.0 distribute-list prefix max24 in!ip prefix-list max24 seq 5 permit 0.0.0.0/0 ge 8 le 24

The following example configuration shows how to conditionally originate a default route (0.0.0.0/0) inRIP when a prefix 10.1.1.0/24 exists in the routing table:

ip prefix-list cond permit 10.1.1.0/24!route-map default-condition permit 10 match ip address prefix-list cond!router rip default-information originate route-map default-condition

The following example shows how to configure BGP to accept routing updates from 192.168.1.1 only,besides filtering on the prefix length:

router bgp 40000 distribute-list prefix max24 gateway allowlist in !ip prefix-list allowlist seq 5 permit 192.168.1.1/32 !

The following example shows how to direct the BGP process to filter incoming updates to the prefix usingname1, and match the gateway (next hop) of the prefix being updated to the prefix list name2, onGigabitEthernet interface 0/0/0:

router bgp 103 distribute-list prefix name1 gateway name2 in gigabitethernet 0/0/0

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Filtering BGP Prefixes Using a Group of PrefixesThe following example shows how to configure BGP to permit routes with a prefix length up to 24 innetwork 192/8:

ip prefix-list abc permit 192.0.0.0/8 le 24

The following example shows how to configure BGP to deny routes with a prefix length greater than 25 in192/8:

ip prefix-list abc deny 192.0.0.0/8 ge 25

The following example shows how to configure BGP to permit routes with a prefix length greater than 8and less than 24 in all address space:

ip prefix-list abc permit 0.0.0.0/0 ge 8 le 24

The following example shows how to configure BGP to deny routes with a prefix length greater than 25 inall address space:


The following example shows how to configure BGP to deny all routes in network 10/8, because any routein the Class A network 10.0.0.0/8 is denied if its mask is less than or equal to 32 bits:

ip prefix-list abc deny 10.0.0.0/8 le 32

The following example shows how to configure BGP to deny routes with a mask greater than 25 in192.168.1.0/24:


The following example shows how to configure BGP to permit all routes:

ip prefix-list abc permit 0.0.0.0/0 le 32

Adding or Deleting Prefix List EntriesYou can add or delete individual entries in a prefix list if a prefix list has the following initial configuration:

ip prefix-list abc deny 0.0.0.0/0 le 7ip prefix-list abc deny 0.0.0.0/0 ge 25ip prefix-list abc permit 192.168.0.0/15

The following example shows how to delete an entry from the prefix list so that 192.168.0.0 is notpermitted, and add a new entry that permits 10.0.0.0/8:

no ip prefix-list abc permit 192.168.0.0/15 ip prefix-list abc permit 10.0.0.0/8

The new configuration is as follows:

ip prefix-list abc deny 0.0.0.0/0 le 7ip prefix-list abc deny 0.0.0.0/0 ge 25ip prefix-list abc permit 10.0.0.0/8

Filtering Traffic Using Community Lists ExamplesThis section contains two examples of the use of BGP communities with route maps.

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The first example shows how the route map named set-community is applied to the outbound updates to theneighbor 172.16.232.50. The routes that pass access list 1 have the special community attribute value no-export. The remaining routes are advertised normally. This special community value automatically preventsthe advertisement of those routes by the BGP speakers in autonomous system 200.

router bgp 100 neighbor 172.16.232.50 remote-as 200 neighbor 172.16.232.50 send-community neighbor 172.16.232.50 route-map set-community out!route-map set-community permit 10 match address 1 set community no-export!route-map set-community permit 20 match address 2

The second example shows how the route map named set-community is applied to the outbound updates toneighbor 172.16.232.90. All the routes that originate from autonomous system 70 have the communityvalues 200 200 added to their already existing values. All other routes are advertised as normal.

route-map bgp 200 neighbor 172.16.232.90 remote-as 100 neighbor 172.16.232.90 send-community neighbor 172.16.232.90 route-map set-community out!route-map set-community permit 10 match as-path 1 set community 200 200 additive!route-map set-community permit 20!ip as-path access-list 1 permit 70$ip as-path access-list 2 permit .*

Filtering Traffic Using AS-path Filters ExampleThe following example shows BGP path filtering by neighbor. Only the routes that pass autonomoussystem path access list 2 will be sent to 192.168.12.10. Similarly, only routes passing access list 3 will beaccepted from 192.168.12.10.

router bgp 200 neighbor 192.168.12.10 remote-as 100 neighbor 192.168.12.10 filter-list 1 out neighbor 192.168.12.10 filter-list 2 in exitip as-path access-list 1 permit _109_ip as-path access-list 2 permit _200$ip as-path access-list 2 permit ^100$ip as-path access-list 3 deny _690$ip as-path access-list 3 permit .*

Filtering Traffic with AS-path Filters Using 4-Byte Autonomous SystemNumbers Examples

Asplain Default Format in Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)SXI1, and LaterReleases

The following example is available in Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE,12.2(33)XNE, 12.2(33)SXI1, and later releases and shows BGP path filtering by neighbor using 4-byte

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autonomous system numbers in asplain format. Only the routes that pass autonomous system path accesslist 2 will be sent to 192.168.3.2.

ip as-path access-list 2 permit ^65536$router bgp 65538 address-family ipv4 unicast neighbor 192.168.3.2 remote-as 65550 neighbor 192.168.3.2 activate neighbor 192.168.3.2 filter-list 2 in end

Asdot Default Format in Cisco IOS Release 12.0(32)S12, and 12.4(24)T

The following example available in Cisco IOS Release 12.0(32)S12, 12.4(24)T, and later releases showsBGP path filtering by neighbor using 4-byte autonomous system numbers in asdot format. Only the routesthat pass autonomous system path access list 2 will be sent to 192.168.3.2.

Note In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, and laterreleases, this example works if you have configured asdot as the default display format using the bgpasnotation dot command.

ip as-path access-list 2 permit ^1\.0$router bgp 1.2 address-family ipv4 unicast neighbor 192.168.3.2 remote-as 1.14 neighbor 192.168.3.2 filter-list 2 in end

Filtering Traffic Using Extended Community Lists with 4-Byte AutonomousSystem Numbers Example

Asplain Default Format in Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)SXI1, and LaterReleases

The following example shows how to filter traffic by creating an extended BGP community list to controloutbound routes. In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE,12.2(33)SXI1, and later releases, extended BGP communities support 4-byte autonomous system numbersin the regular expressions in asplain by default. Extended community attributes are used to configure, filter,and identify routes for VRF instances and MPLS VPNs. The ip extcommunity-listcommand is used toconfigure named or numbered extended community lists. All of the standard rules of access lists apply to

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the configuration of extended community lists. Regular expressions are supported by the expanded range ofextended community list numbers.

Figure 10 BGP Topology for Filtering Traffic Using Extended Community Lists with 4-Byte Autonomous SystemNumbers in Asplain Format


In this exam the figure above is configured with an extended named community list to specify that the BGPpeer at 192.1681.2 is not sent advertisements about any path through or from the 4-byte autonomoussystem 65550. The IP extended community-list configuration mode is used, and the ability to resequenceentries is shown.

ip extcommunity-list expanded DENY65550 10 deny _65550_ 20 deny ^65550 .* resequence 50 100 exitrouter bgp 65538 network 172.17.1.0 mask 255.255.255.0 address-family ipv4 unicast neighbor 192.168.3.2 remote-as 65550 neighbor 192.168.1.2 remote-as 65536 neighbor 192.168.3.2 activate neighbor 192.168.1.2 activate endshow ip extcommunity-list DENY65550

Asdot Default Format in Cisco IOS Release 12.0(32)S12, and 12.4(24)T

The following example shows how to filter traffic by creating an extended BGP community list to controloutbound routes. In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and later releases, extended BGPcommunities support 4-byte autonomous system numbers in the regular expressions in asdot format only.Extended community attributes are used to configure, filter, and identify routes for VRF instances andMPLS VPNs. The ip extcommunity-listcommand is used to configure named or numbered extended

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community lists. All of the standard rules of access lists apply to the configuration of extended communitylists. Regular expressions are supported by the expanded range of extended community list numbers.

Note In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SXI1, and later releases, this example works ifyou have configured asdot as the default display format using the bgp asnotation dot command.

Figure 11 BGP Topology for Filtering Traffic Using Extended Community Lists with 4-Byte Autonomous SystemNumbers in Asdot Format


In this exam the figure above is configured with an extended named community list to specify that the BGPpeer at 192.1681.2 is not sent advertisements about any path through or from the 4-byte autonomoussystem 65550. The IP extended community-list configuration mode is used, and the ability to resequenceentries is shown.

ip extcommunity-list expanded DENY114 10 deny _1\.14_ 20 deny ^1\.14 .* resequence 50 100 exitrouter bgp 1.2 network 172.17.1.0 mask 255.255.255.0 address-family ipv4 unicast neighbor 192.168.3.2 remote-as 1.14 neighbor 192.168.1.2 remote-as 1.0 neighbor 192.168.3.2 activate neighbor 192.168.1.2 activate endshow ip extcommunity-list DENY114

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Filtering Traffic Using a BGP Route Map ExampleThe following example shows how to use an address family to configure BGP so that any unicast andmulticast routes from neighbor 10.1.1.1 are accepted if they match access list 1:

route-map filter-some-multicast match ip address 1 exitrouter bgp 65538 neighbor 10.1.1.1 remote-as 65537 address-family ipv4 unicast neighbor 10.1.1.1 activate neighbor 10.1.1.1 route-map filter-some-multicast in exit exitrouter bgp 65538 neighbor 10.1.1.1 remote-as 65537 address-family ipv4 multicast neighbor 10.1.1.1 activate neighbor 10.1.1.1 route-map filter-some-multicast in end

Filtering Traffic Using Continue Clauses in a BGP Route Map ExamplesThe following example shows continue clause configuration in a route map sequence.

Note Continue clauses in outbound route maps are supported only in Cisco IOS Release 12.0(31)S, 12.2(33)SB,12.2(33)SRB, 12.2(33)SXI, 12.4(4)T, and later releases.

The first continue clause in route map entry 10 indicates that the route map will go to route map entry 30 ifa successful matches occurs. If a match does not occur, the route map will "fall through" to route map entry20. If a successful match occurs in route map entry 20, the set action will be executed and the route mapwill not evaluate any additional route map entries. Only the first successful match ip address clause issupported.

If a successful match does not occur in route map entry 20, the route map will "fall through" to route mapentry 30. This sequence does not contain a match clause, so the set clause will be automatically executedand the continue clause will go to the next route map entry because a sequence number is not specified.

If there are no successful matches, the route map will "fall through" to route map entry 30 and execute theset clause. A sequence number is not specified for the continue clause so route map entry 40 will beevaluated.

There are two behaviors that can occur when the same set command is repeated in subsequent continueclause entries. For set commands that configure an additive or accumulative value (for example, setcommunity additive, set extended community additive, and set as-path prepend), subsequent values areadded by subsequent entries. The following example illustrates this behavior. After each set of matchclauses, a set as-path prepend command is configured to add an autonomous system number to the as-path. After a match occurs, the route map stops evaluating match clauses and starts executing the setclauses, in the order in which they were configured. Depending on how many successful match clausesoccur, the as-path is prepended by one, two, or three autonomous system numbers.

route-map ROUTE-MAP-NAME permit 10 match ip address 1 match metric 10 set as-path prepend 10 continue 30 !

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route-map ROUTE-MAP-NAME permit 20 match ip address 2 match metric 20 set as-path prepend 10 10 !route-map ROUTE-MAP-NAME permit 30 set as-path prepend 10 10 10 continue !route-map ROUTE-MAP-NAME permit 40 match community 10:1 set local-preference 104

In this example, the same set command is repeated in subsequent continue clause entries but the behavior isdifferent from the first example. For setcommands that configure an absolute value, the value from the lastinstance will overwrite the previous value(s). The following example illustrates this behavior. The setclause value in sequence 20 overwrites the set clause value from sequence 10. The next hop for prefixesfrom the 172.16/16 network is set to 10.2.2.2 and not 10.1.1.1.

ip prefix-list 1 permit 172.16.0.0/16 ip prefix-list 2 permit 192.168.1.0/24 route-map RED permit 10 match ip address prefix-list 1 set ip next hop 10.1.1.1 continue 20 exit route-map RED permit 20 match ip address prefix-list 2 set ip next hop 10.2.2.2 end

Note Route maps have a linear behavior and not a nested behavior. Once a route is matched in a route mappermit entry with a continue command clause, it will not be processed by the implicit deny at the end of theroute-map. The following example illustrates this case.

In the following example, when routes match an as-path of 10, 20, or 30, the routes are permitted and thecontinue clause jumps over the explicit deny clause to process the match ip address prefix list. If a matchoccurs here, the route metric is set to 100. Only routes that do not match an as-path of 10, 20, or 30 and domatch a community number of 30 are denied. To deny other routes, you must configure an explicit denystatement.

route-map test permit 10 match as-path 10 20 30 continue 30 exitroute-map test deny 20 match community 30 exitroute-map test permit 30 match ip address prefix-list 1 set metric 100 exit

Where to Go Next• To configure advanced BGP feature tasks, proceed to the "Configuring Advanced BGP Features"

module.• To configure BGP neighbor session options, proceed to the "Configuring BGP Neighbor Session

Options" module.

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• To configure internal BGP tasks, proceed to the "Configuring Internal BGP Features" module.

Additional ReferencesThe following sections provide references related to connecting to a service provider using external BGP.

Related Documents

Related Topic Document Title

Cisco IOS commands Cisco IOS Master Commands List, All Releases

BGP commands: complete command syntax,command mode, defaults, command history, usageguidelines, and examples

Cisco IOS IP Routing: BGP Command Reference

BGP overview "Cisco BGP Overview" module

Configuring basic BGP tasks "Configuring a Basic BGP Network" module

BGP fundamentals and description Large-Scale IP Network Solutions , Khalid Razaand Mark Turner, Cisco Press, 2000

Implementing and controlling BGP in scalablenetworks

Building Scalable Cisco Networks , CatherinePaquet and Diane Teare, Cisco Press, 2001

Interdomain routing basics Internet Routing Architectures , Bassam Halabi,Cisco Press, 1997

Standards

Standard Title

MDT SAFI MDT SAFI

MIBs

MIB MIBs Link

CISCO-BGP4-MIB To locate and download MIBs for selectedplatforms, Cisco IOS releases, and feature sets, useCisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC Title

RFC 1772 Application of the Border Gateway Protocol in theInternet

Filtering Traffic Using Continue Clauses in a BGP Route Map ExamplesAdditional References

85

http://www.cisco.com/en/US/docs/ios/mcl/allreleasemcl/all_book.html

http://www.iana.org/assignments/safi-namespace

http://www.cisco.com/go/mibs

RFC Title

RFC 1773 Experience with the BGP Protocol

RFC 1774 BGP-4 Protocol Analysis

RFC 1930 Guidelines for Creation, Selection, andRegistration of an Autonomous System (AS)

RFC 2519 A Framework for Inter-Domain Route Aggregation

RFC 2858 Multiprotocol Extensions for BGP-4

RFC 2918 Route Refresh Capability for BGP-4

RFC 3392 Capabilities Advertisement with BGP-4

RFC 4271 A Border Gateway Protocol 4 (BGP-4)

RFC 4893 BGP Support for Four-Octet AS Number Space

RFC 5396 Textual Representation of Autonomous system(AS) Numbers

RFC 5398 Autonomous System (AS) Number Reservation forDocumentation Use

Technical Assistance.

Description Link

The Cisco Support website provides extensiveonline resources, including documentation and toolsfor troubleshooting and resolving technical issueswith Cisco products and technologies.

To receive security and technical information aboutyour products, you can subscribe to variousservices, such as the Product Alert Tool (accessedfrom Field Notices), the Cisco Technical ServicesNewsletter, and Really Simple Syndication (RSS)Feeds.

Access to most tools on the Cisco Support websiterequires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Connecting to a Service ProviderUsing External BGP

The following table provides release information about the feature or features described in this module.This table lists only the software release that introduced support for a given feature in a given software

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http://www.cisco.com/public/support/tac/home.shtml

http://www.cisco.com/public/support/tac/home.shtml

release train. Unless noted otherwise, subsequent releases of that software release train also support thatfeature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support.To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 5 Feature Information for Connecting to a Service Provider Using External BGP

Feature Name Releases Feature Configuration Information

BGP Increased Support ofNumbered AS-Path Access Liststo 500

12.0(22)S 12.2(15)T 12.2(18)S12.2(18)SXD 12.2(27)SBC15.0(1)S

The BGP Increased Support ofNumbered AS-Path Access Liststo 500 feature increases themaximum number of autonomoussystems access lists that can beconfigured using the ip as-pathaccess-list command from 199 to500.

BGP Named Community Lists 12.2(8)T 12.2(14)S 15.0(1)S The BGP Named CommunityLists feature introduces a newtype of community list called thenamed community list. The BGPNamed Community Lists featureallows the network operator toassign meaningful names tocommunity lists and increases thenumber of community lists thatcan be configured. A namedcommunity list can be configuredwith regular expressions and withnumbered community lists. Allrules of numbered communitiesapply to named community listsexcept that there is no limitationon the number of communityattributes that can be configuredfor a named community list.

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http://www.cisco.com/go/cfn


BGP Prefix-Based OutboundRoute Filtering

12.0(22)S 12.2(4)T 12.2(14)S15.0(1)S

The BGP Prefix-Based OutboundRoute Filtering feature uses BGPORF send and receive capabilitiesto minimize the number of BGPupdates that are sent betweenBGP peers. Configuring thisfeature can help reduce theamount of system resourcesrequired for generating andprocessing routing updates byfiltering out unwanted routingupdates at the source. Forexample, this feature can be usedto reduce the amount ofprocessing required on a routerthat is not accepting full routesfrom a service provider network.

BGP Route-Map Continue 12.0(24)S 12.2(18)S12.2(18)SXD 12.2(27)SBC12.3(2)T 15.0(1)S Cisco IOS XE3.1.0SG

The BGP Route-Map Continuefeature introduces the continueclause to BGP route mapconfiguration. The continueclause allows for moreprogrammable policyconfiguration and route filteringand introduces the capability toexecute additional entries in aroute map after an entry isexecuted with successful matchand set clauses. Continue clausesallow the network operator toconfigure and organize moremodular policy definitions so thatspecific policy configurationsneed not be repeated within thesame route map.

BGP Route-Map ContinueSupport for an Outbound Policy

12.0(31)S 12.2(33)SB12.2(33)SRB 12.2(33)SXI12.4(4)T 15.0(1)S Cisco IOS XE3.1.0SG

The BGP Route-Map ContinueSupport for an Outbound Policyfeature introduces support forcontinue clauses to be applied tooutbound route maps.


88


BGP Route-Map Policy ListSupport

12.0(22)S 12.2(15)T 12.2(18)S12.2(18)SXD 12.2(27)SBC15.0(1)S

The BGP Route-Map Policy ListSupport feature introduces newfunctionality to BGP route maps.This feature adds the capabilityfor a network operator to grouproute map match clauses intonamed lists called policy lists. Apolicy list functions like a macro.When a policy list is referencedin a route map, all of the matchclauses are evaluated andprocessed as if they had beenconfigured directly in the routemap. This enhancementsimplifies the configuration ofBGP routing policy in medium-size and large networks because anetwork operator canpreconfigure policy lists withgroups of match clauses and thenreference these policy lists withindifferent route maps. The networkoperator no longer needs tomanually reconfigure eachrecurring group of match clausesthat occur in multiple route mapentries.

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BGP Support for 4-Byte ASN 12.0(32)S12 12.0(32)SY812.0(33)S3 12.2(33)SRE12.2(33)XNE 12.2(33)SXI112.4(24)T 15.0(1)S Cisco IOSXE 3.1.0SG

The BGP Support for 4-ByteASN feature introduced supportfor 4-byte autonomous systemnumbers. Because of increaseddemand for autonomous systemnumbers, in January 2009 theIANA will start to allocate 4-byteautonomous system numbers inthe range from 65536 to4294967295.

In Cisco IOS Release12.0(32)SY8, 12.0(33)S3,12.2(33)SRE, 12.2(33)XNE, and12.2(33)SXI1, the Ciscoimplementation of 4-byteautonomous system numbers usesasplain as the default regularexpression match and outputdisplay format for autonomoussystem numbers, but you canconfigure 4-byte autonomoussystem numbers in both theasplain format and the asdotformat as described in RFC 5396.To change the default regularexpression match and outputdisplay of 4-byte autonomoussystem numbers to asdot format,use the bgp asnotation dotcommand.

In Cisco IOS Release12.0(32)S12, and 12.4(24)T, theCisco implementation of 4-byteautonomous system numbers usesasdot as the only configurationformat, regular expression match,and output display, with noasplain support.

The following commands wereintroduced or modified by thisfeature: bgp asnotation dot, bgpconfederation identifier, bgpconfederation peers, all clear ipbgpcommands that configure anautonomous system number, ipas-path access-list, ipextcommunity-list, matchsource-protocol, neighbor local-


90


as, neighbor remote-as,neighbor soo, redistribute (IP),router bgp, route-target, set as-path, set extcommunity, setorigin, soo, all show ip bgpcommands that display anautonomous system number, andshow ip extcommunity-list.

BGP Support for NamedExtended Community Lists

12.2(25)S 12.2(27)SBC12.2(33)SRA 12.2(33)SXH12.3(11)T 15.0(1)S

The BGP Support for NamedExtended Community Listsfeature introduces the ability toconfigure extended communitylists using names in addition tothe existing numbered format.

BGP Support for SequencedEntries in Extended CommunityLists

12.2(25)S 12.2(27)SBC12.2(33)SRA 12.2(33)SXH12.3(11)T 15.0(1)S

The BGP Support for SequencedEntries in Extended CommunityLists feature introduces automaticsequencing of individual entriesin BGP extended communitylists. This feature also introducesthe ability to remove orresequence extended communitylist entries without deleting theentire existing extendedcommunity list.

BGP 4 Prefix Filter and InboundRoute Maps

Cisco IOS XE 3.1.0SG

Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and othercountries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third partytrademarks mentioned are the property of their respective owners. The use of the word partner does notimply a partnership relationship between Cisco and any other company. (1005R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to beactual addresses and phone numbers. Any examples, command display output, network topology diagrams,and other figures included in the document are shown for illustrative purposes only. Any use of actual IPaddresses or phone numbers in illustrative content is unintentional and coincidental.

© 2011 Cisco Systems, Inc. All rights reserved.

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